Preservative steel plate having high resistance weldability, corrosion resistance and press formability for automobile fuel tanks

ABSTRACT

A coating aluminized steel sheet suitable for fuel tanks, which comprises (a) a steel sheet, (b) an aluminized-plating layer formed on one or both sides of the steel sheet and based on aluminum or an aluminum alloy containing 2-15 wt% silicon, and (c) a coating layer formed on at least one of the aluminizing layers and selected from the group consisting of i) a resin chromate film having a film thickness of 0.1-2 μm and containing a resin and a chromic acid compound, with the resin/metal chromium weight ratio in the range of 0.5-18, ii) an inorganic-based chromate film with the coating layer formed to 10-200 mg/m 2  in terms of metallic chromium, which comprises 100 parts by weight of a chromic acid compound in terms of metallic chromium and 100-1000 parts by weight of colloidal silica, and further comprises any one or more of 100-600 parts by weight of a phosphoric acid compound, 10-200 parts by weight of a phosphonic acid or phosphonic acid salt compound and less than 50 parts by weight of a resin, and iii) an inorganic-based chromate film with a coating amount of at least 10 mg/m 2  and less than 35 mg/m 2  in terms of metallic chromium. There are provided automobile fuel tanks with excellent durability, forming formability and weldability, and a seam welding process for fuel tanks.

TECHNICAL FIELD

The present invention relates to a rustproof steel sheet for automobilefuel tanks which has excellent resistance weldability, corrosionresistance and press formability. The invention further relates to anautomobile fuel tank with excellent corrosion resistance and to a seamwelding process for automobile fuel tanks.

BACKGROUND ART

Automobile fuel tanks usually have a final design which is in conformitywith the design of automobile bodies, and their shapes therefore tend tobe very complicated. Their structure includes, as shown in FIG. 1, afuel supply opening 3, a fuel supply pump (not shown), a fuel hose 4,the fuel hose 4 serving to return excess fuel 6, separators 5 to preventthe sound of fuel waves, etc. The fuel tank body 1 consists of a pair ofbowl-shaped molds formed into an integral whole by seam-welding theflange members 2. Each of the parts are bonded by spot welding,soldering or brazing.

This fuel tank is an important member of the automobile from a safetystandpoint, and it is required to possess the features of sufficientcorrosion resistance against fuel, leakproofness and impermeability tofuel, and also low fatigue after forming and resistance to cracking byimpacts. The corrosion resistance is of course to eliminate the concernof corrosion holes, but it is also important in terms of preventingproduction of abundant corrosion products which lead to clogging of thefilter at the inlet of the fuel pump in the fuel tank.

Various modifications have been made in the materials, manufacture andmanufacturing processes to obtain fuel tanks with such properties. As aresult of modifications in the materials, it has become common to employfuel tanks made of Pb—Sn plated steel sheets which have sufficientcorrosion resistance against fuel, low generation of corrosion productsand easier welding and soldering suitability for better productionefficiency (Japanese Examined Patent Publication No. 57-61833). However,Pb is a metal which is detrimental to the environment, as is well known.Also, while Pb—Sn plated steel sheets are well suited for soldering andbrazing as mentioned above, the soldering component is an Sn—Pn systemwhich of course contains Pb. Consequently, with recent demands for fueltanks which employ absolutely no Pb, fuel tanks made of Al—Si basedalloy plated (hereunder, “aluminized”) steel sheets have become a focusof study as candidate substitutes.

Aluminized steel sheets are one type of material which utilize no Pb andhave satisfactory corrosion resistance and workability. Aluminizingforms a stable oxide film on the surface, and therefore providessatisfactory corrosion resistance against not only gasoline but alsoalcohol and organic acids produced by degradation of gasoline. However,several problems arise when aluminized steel sheets are used as fueltank materials. One of these is workability, and aluminized steel sheets(especially hot dip aluminized steel sheets) are susceptible to platinglayer peel and plating layer cracks originating from sections of veryhard Fe—Al—Si intermetallic compounds (hereunder referred to as the“alloy layer”) produced at the interface between the coated layer andthe steel sheet. The present inventors have dealt with this issue inJapanese Patent Application No. 7-329193, disclosing that it can beovercome by adjusting the cooling rate and reheating after plating.Another problem is weldability. Specifically, although aluminized steelsheets are suitable for resistance welding such as spot welding and seamwelding, the coated Al metal has high affinity for Cu which is usuallyused as the electrode, and forms brittle Al—Cu or Al—Cu—Fe alloys on theelectrode surface during welding, thus resulting in the problem ofgradual loss during continuous operation and early welding defects.

Aluminized steel sheets have been conventionally used after beingsubjected to chromate treatment, mainly with chromic acid and silica,for the purpose of improving corrosion resistance, and disclosedinstances thereof include Japanese Examined Patent Publication No.4-68399, Japanese Unexamined Patent Publication No. 58-6976, JapaneseUnexamined Patent Publication No. 58-48679 and Japanese UnexaminedPatent Publication No. 60-56072. All of these methods, however,contribute little to improvement in continuous operation because thereactions with the electrode are virtually the same as with untreatedmaterials. The process of Japanese Examined Patent Publication No.4-68399 is characterized by forming the coating to 35-70 mg/m² in termsof Cr, but although corrosion resistance of the fuel tank is achievedwith this amount of coating, there is a disadvantage for spot weldingand seam welding, in that the Al in the plating layer tends to formalloys with the electrode Cu as with untreated materials, so that theelectrode tip becomes alloyed during continuous operation thusshortening the life of the electrode. In addition, if the brazingmaterial is not carefully selected, the wettability of the brazingmaterial will be lower resulting in the problem of a more difficultbrazing operation, and tanks with brazed pipes, etc. will be difficultto manufacture. Japanese Unexamined Patent Publications No. 58-6976 andNo. 58-48679 disclose processes characterized by the amount of chromatecoating of 5-40 mg/m² in terms of Cr and organic silicon water repellenttreatment, but in addition to the same problems with resistance weldingas Japanese Examined Patent Publication No. 4-68399, the corrosionresistance for fuel tanks is poor at less than 10 mg/M² even withorganic silicon water repellents, and the corrosion resistance againstorganic acids produced by degradation of gasoline fuel is insufficient.Also, as in Japanese Examined Patent Publication No. 4-68399, despitethe improved corrosion resistance at 35 mg/m² and greater, failure tocarefully select the brazing material will result in a lower wettabilityof the brazing material, thus complicating the brazing operation.Another disadvantage is that in spot welding and seam welding, the Al inthe plating layer tends to form alloys with the electrode Cu as withuntreated materials, so that the electrode tip becomes alloyed duringcontinuous operation thus shortening the life of the electrode. Theprocess in Japanese Unexamined Patent Publication No. 60-56072 ischaracterized by the amount of chromate coating of less than 10 mg/m²,and thus its drawback is that it cannot provide the weldability or thecorrosion resistance required for fuel tanks. With these conventionaltechniques, it has been difficult to satisfactorily achieve theresistance weldability, continuous operation and corrosion resistancerequired for production of fuel tanks.

It is an object of the present invention to provide an aluminized steelsheet for a fuel tank material, which improves resistance weldabilityover rustproof steel sheets for fuel tanks for which conventionalaluminized steel sheets have not been suitable, as well as satisfactorypress formability and corrosion resistance.

It is another object of the invention to provide a novel fuel tank whichis environmentally friendly by not using Pb, and which has excellentcorrosion resistance in environments of gasoline and other fuels.

It is yet another object of the invention to provide a seam weldingprocess by which it is possible to achieve improved resistanceweldability over rustproof steel sheets for fuel tanks for whichconventional aluminized steel sheets have not been suitable, as well ascontinuous operation.

DISCLOSURE OF THE INVENTION

The present invention provides the following in order to attain theobjects described above.

(1) A coating aluminized steel sheet suitable for fuel tanks, whichcomprises

(a) a steel sheet,

(b) an aluminizing layer formed on one or both sides of the steel sheetand based on aluminum or an aluminum alloy containing 2-15 wt % silicon,and

(c) a coating layer formed on at least one of the aluminizing layers andselected from the group consisting of

i) an organic and inorganic composite chromate film having a filmthickness of 0.1-2 μm and containing a resin and a chromic acidcompound, with the resin/metal chromium weight ratio in the range of0.5-18,

ii) an inorganic-based chromate film A with the coating layer formed to10-200 mg/m² in terms of metallic chromium, which comprises 100 parts byweight of chromic acid in terms of metallic chromium and 100-1000 partsby weight of colloidal silica, and further comprises at least oneselected from the group consisting of 100-600 parts by weight of aphosphoric acid compound, 10-200 parts by weight of a phosphonic acid orphosphonic acid salt compound and less than 50 parts by weight of anorganic resin, and

iii) an inorganic-based chromate film B with a coating amount of atleast 10 mg/m² and less than 35 mg/m² in terms of metallic chromium.

(2) A coating aluminized steel sheet according to (1) above, wherein thealuminizing layer is formed to 60 g/m² or less.

(3) A coating aluminized steel sheet according to (1) or (2) above,wherein the composite chromate film further contains 0.5-20 wt % of alubricant.

(4) A coating aluminized steel sheet according to (1), (2) or (3) above,wherein the composite chromate film further contains 100-600 parts byweight of a phosphoric acid compound and 100-1000 parts by weight ofcolloidal silica with respect to 100 parts by weight of metallicchromium.

(5) A coating aluminized steel sheet according to (4) above, wherein thecomposite chromate film further contains 10-200 parts by weight of aphosphonic acid or phosphonic acid salt compound with respect to 100parts by weight of metallic chromium.

(6) A coating aluminized steel sheet according to any of (1) to (5)above, which has the aluminizing layer on both sides of the steel sheetand which has the composite chromate film on the aluminizing layers onboth sides.

(7) A coating aluminized steel sheet according to (1) or (2) above,which has the aluminizing layer on both sides of the steel sheet, andwhich has the inorganic-based chromate film A) on the aluminizing layerson both sides.

(8) A coated aluminized steel sheet according to any of (1) to (5)above, which has the aluminizing layer on both sides of the steel sheetand which has the composite chromate film on the aluminizing layer onone side and an inorganic-based chromate film C with a coating amount of200 mg/m² or less in terms of metallic chromium on the aluminizing layeron the other side.

(9) A coating aluminized steel sheet according to (8) above, wherein theinorganic-based chromate film C formed on the aluminizing layer furthercontains at least one selected from the group consisting of phosphoricacid compounds, phosphonic acid and phosphonic acid salt compounds, andless than 50 parts by weight of a resin with respect to 100 parts byweight of metallic chromium.

(10) A coating aluminized steel sheet according to (1) or (8) above,which has an inorganic-based chromate film C at 100 mg/m² or less interms of metallic chromium between the aluminizing layer and thecomposite chromate film.

(11) A coating aluminized steel sheet according to (10), wherein theinorganic-based chromate film C formed between the aluminizing layer andthe composite chromate film further contains at least one selected fromthe group consisting of phosphoric acid compounds, phosphonic acid andphosphonic acid salt compounds, and less than 10 parts by weight of aresin with respect to 100 parts by weight of metallic chromium.

(12) A coating aluminized steel sheet according to (1) above, which hasthe aluminizing layer on both sides of the steel sheet, and which hasthe inorganic-based chromate film B formed to 10-35 mg/m² in terms ofmetallic chromium on the aluminizing layers on both sides.

(13) A coating aluminized steel sheet according to any of (1) to (5)above, which has the aluminizing layer on both sides of the steel sheet,and which has the composite chromate film on the aluminizing layer onone side and an inorganic resin film with a thickness of 0.1-2.0 μm onthe aluminizing layer on the other side.

(14) A coating aluminized steel sheet according to (13) above, which hasan inorganic-based chromate film C at 100 mg/m² or less in terms ofmetallic chromium between the aluminizing layer and the compositechromate film and/or the organic resin film.

(15) A coating aluminized steel sheet according to (14), wherein theinorganic-based chromate film C formed on the aluminizing layer furthercontains at least one selected from the group consisting of phosphoricacid compounds, phosphonic acid and phosphonic acid salt compounds, andless than 50 parts by weight of a resin with respect to 100 parts byweight of metallic chromium.

(16) A coating aluminized steel sheet according to (1) above, which hasthe aluminizing layer on both sides of the steel sheet and which has theinorganic-based chromate film B) on the aluminizing layer on one sideand an organic-based resin film on the aluminizing layer on the otherside.

(17) A coating aluminized steel sheet according to (16), wherein theinorganic-based chromate film B) is formed to 200 mg/m² in terms ofmetallic chromium.

(18) A coating aluminized steel sheet according to (17), wherein theinorganic-based chromate film B formed on the aluminizing layer furthercontains at least one selected from the group consisting of phosphoricacid compounds, phosphonic acid and phosphonic acid salt compounds, andless than 50 parts by weight of a resin with respect to 100 parts byweight of metallic chromium.

(19) A coating aluminized steel sheet according to (17) above, which hasan inorganic-based chromate film C at 100 mg/m² or less in terms ofmetallic chromium between the aluminizing layer and the organic resinfilm.

(20) A coating aluminized steel sheet according to (19), wherein theinorganic-based chromate film C formed between the aluminizing layer andthe organic resin film further contains at least one selected from thegroup consisting of phosphoric acid compounds, phosphonic acid andphosphonic acid salt compounds, and less than 5 parts by weight of aresin with respect to 100 parts by weight of metallic chromium.

(21) A fuel tank produced with a coating aluminized steel sheetaccording to any of (1) to (20) above.

According to the invention, the following are particularly provided asautomobile fuel tanks with excellent corrosion resistance.

(22) An automobile fuel tank wherein a pair of bowl-shaped bodies withflanges are integrated by continuous seam-welding of the flangesubstances, the automobile fuel tank being characterized in that thematerials of which the bowl-shaped bodies are made are coatingaluminized steel sheets which consist of aluminized steel sheets eachhaving on one or both sides an aluminizing layer based on aluminum or analuminum alloy containing 2-13 wt % silicon, and having a resin coatingon the uppermost surface of the inner and/or outer side.

(23) An automobile fuel tank according to (22) above, wherein the resincoating is an organic and inorganic composite chromate film consistingof a mixture of a resin and a chromic acid compound.

(24) An automobile fuel tank according to (22) above, wherein the resincoating has a thickness of 0.1-2 μm.

(25) An automobile fuel tank according to (22) above, wherein thecoating aluminized steel sheets are coating aluminized steel sheetsaccording to any one of (1) to (20) above.

The present invention still further provides the following as seamwelding processes for fuel tanks.

(26) A seam welding process for fuel tanks, in which two coatingaluminized steel sheets are combined which are aluminized steel sheetseach having formed on one or both sides an aluminizing layer based onaluminum or an aluminum alloy containing 2-13 wt % silicon and having aresin coating formed on the one or both sides thereof, wherein thecoating aluminized steel sheets have their aluminizing layer at least onthe side corresponding to the inner side of the fuel tank, a resin filmis provided on at least one of the steel sheet surfaces at the sidewhere the steel sheets meet and/or on at least one of the steel sheetsurfaces at the side where it will contact with an electrode wheel, andthe two combined steel sheets are then seam welded between a pair ofelectrode wheels.

(27) The process according to (26) above, wherein the resin filmcontains a chromic acid compound at 10-200 mg/m² in terms of metallicchromium.

(28) The process according to (27) above, wherein the resin film has athickness of 0.1-2 μm.

(29) The process according to (26) above, wherein the resin film formedon the surface of the aluminized steel sheet is an organic and inorganiccomposite chromate film according to (1) above.

(30) The process according to (26) above, wherein the coating aluminizedsteel sheet is a coating aluminized steel sheet according to any one of(1) to (11) or (13)-(20) above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an overview of the structure of an automobile fuel tank.

FIG. 2 is a bar graph showing the contact resistance values for aninorganic chromate film according to the prior art and an organic andinorganic composite chromate film according to the invention.

FIG. 3 is a graph showing stearic acid lubricant additive contents andBowden friction coefficients, with the blackened state of tape appliedto the external side after cylindrical drawing.

FIG. 4 is a vertical cross-sectional view of the lower part of anautomobile fuel tank.

FIGS. 5A-5C are illustrations of seam welding of automobile fuel tanks.

FIG. 6A is a bar graph showing the contact resistance values for theresin coated materials and untreated materials shown in FIGS. 6B-6D.

BEST MODE FOR CARRYING OUT THE INVENTION Coated Aluminized Steel Sheet

The coated aluminized steel sheet of the invention is characterized inthat on the surface of one or both sides of an aluminized steel sheetthere is formed i) an organic and inorganic composite chromate film, ii)an inorganic-based chromate film A or iii) an inorganic chromate film B,which are explained below, and it is particularly suitable for use inautomobile fuel tanks.

The composition of the plated sheet used is not particularly restricted.However, IF steel (ultra low-carbon sheet steel) which has excellentworkability is preferred only at the sections which require highworkability, and B (boron) is preferably added to the steel sheet at afew ppm to ensure airtightness and secondary workability after welding.

The process for producing the steel sheet may be a common employed one.For example, the steel component may be modified by conversion-vacuumdegassing processing to form an ingot, and a steel billet may beproduced therefrom by continuous founding, etc. and then hot rolled. Theconditions for hot rolling or subsequent cold rolling will affect thedeep drawing properties of the steel sheet. For especially superior deepdrawing properties, the heating temperature during hot rolling should beas low as about 1150° C., the finishing temperature for hot rolling aslow as about 800° C., the coiling temperature as high as 600° C. orabove, and the cold rolling draft as high as about 80%.

The reasons for the restrictions on the aluminizing layer will now beexplained. The plating layer may be Al alone, but Si is preferablyadded. Regarding the Si content in the plated coating layer, thiselement is usually added at about 10% for the purpose of thinning thealloy layer. As mentioned above, alloy layers produced by hot dipaluminizing are extremely hard and brittle and thus tend to formbreaking origins, thus also impairing the ductility of the steel sheetitself. Even with common alloy layers of about 2-3 μm, the ductility isreduced by about 3 points. Consequently, a thinner alloy layer willfunction more advantageously when worked. If the Si is not added to atleast 2% the alloy layer-reducing effect will be weaker, and if added toover 15% the effect will be saturated, while the tendency for Si to actas an electrochemical cathode will lower the corrosion resistance of theplating layer with the greater Si content. For these reasons, the Sicontent is limited to 2-15%. The preferred lower limit is 3%, and thepreferred upper limit is 13%.

According to the invention, hot dip aluminizing is preferred.

Furthermore, while a greater plating amount of plating will improve thecorrosion resistance, it will reduce the plating adhesion andweldability. When applying hot dip aluminized steel sheets as fuel tankmaterials which require different types of welding, it is important toensure weldability and therefore the maximum amount of plating is 60g/m² per side. It is preferably no greater than 50 g/m² and morepreferably no greater than 40 g/m², per side. There are no particularrestrictions on the other conditions for the aluminizing. However, asmaller alloy layer thickness is preferred, as mentioned above.

After-processing which follows the hot dip plating may include zerospangling (minimized spangling) for a uniform outer appearance after hotdip plating, annealing for modification of the plating, and temperedrolling for adjustment of the surface condition and quality, butaccording to the invention any process may be applied without anylimitation to these.

(First embodiment)

According to a first embodiment of the coating aluminized steel sheet ofthe invention, an organic and inorganic composite chromate film(hereunder referred to simply as “composite chromate film”) is formed onthe aluminizing layer on one or both sides of the aluminized steelsheet.

Here, an organic and inorganic composite chromate film is a film whichis a mixture of an organic resin and an inorganic chromic acid compound,and the term encompasses a wide range including films modified byaddition of resins, which have the basic properties of a resin film butwith a chromic acid compound (chromic acid, chromic anhydride, chromicacid salt, chromic acid ester, chromic acid ion compound, etc. butespecially a chromic acid salt) dispersed in a resin matrix, so thatproperties similar to those of an inorganic-based chromate film areretained.

The present inventors have conducted much research on after-processingof aluminized steel sheets with excellent weldability, formability andcorrosion resistance, and as a result we have resolved theaforementioned issue of continuous operation during welding by suitablyforming on the surface a film having a chromate film structure whichcomprises an organic and inorganic composite chromate film consisting ofan appropriate combination of an inorganic component such as a chromicacid compound or silica and an organic component such as a resin, and wehave found that such products have excellent properties for fuel tanks.

As mentioned above, the steel sheet coating metal, Al, readily reactswith the Cu electrode, resulting in the problem of more rapid electrodeloss and poorer continuous operation. Accordingly, there are 2 importantobjects for improved continuous operation: minimizing the electrode lossand increasing the contact resistance value between the steel sheets inorder to form more efficient nuggets. The present inventors havediscovered that an organic and inorganic composite chromate film can beeffectively employed for this purpose, and the present invention hasthus been completed.

Although the mechanism is not yet completely understood, it is theorizedthat there is an effect of Cr in addition to the contact resistanceincrease by mere resin application. The organic resin-rich compositechromate treatment employs a chromic acid compound in the form of anaqueous solution, so that Cr is uniformly distributed throughout thecoating, and this is also believed to contribute to the improvedweldability.

In other words, a film constructed only with the inorganic components ofchromic acid compounds and silica, having an amount of coating as ofconventional chromate treatment as shown in FIG. 3, gives a contactresistance value between steel sheets, which is not unlike that ofuntreated materials, and thus like untreated materials, the plating Aland the electrode Cu react during welding so that there is no increasein the usable life of the electrode. Conversely, if the amount ofcoating is increased, a harder and brittler inorganic film results, andtherefore despite the higher contact resistance value, local breakage ofthe film occurs and the contact resistance value varies drasticallybecause of non-uniformity of weld current passing points between sheetand electrode, so that no reduction in electrode loss can be expected.Another problem is that local over-current passing between the sheet andelectrode tends to produce explosion.

In contrast, addition of an organic component is believed to increasethe tenacity of the film and eliminate local breakage of the film andvariations in forming the weld current passing point, thus facilitatingformation of uniform weld current passing points between sheet andelectrode as compared with films comprising only inorganic components.Thus, even though a high contact resistance value between sheets isobtained, the contact resistance value between sheet and electrode isuniformly low (FIG. 2), thus providing satisfactory nugget-formingproperties and an electrode loss minimizing effect. These effects aregreatest when both sides are treated, but an effect is still exhibitedwhen one side is treated.

FIG. 2 shows the contact resistance values between upper electrode andsheet, the contact resistance values between sheet and sheet and thecontact resistance values between sheet and lower electrode fordifferent sample steel sheets, and the samples are the following listedin Examples 29-50.

Inorganic chromate 1: Comparative Example I solution, the amount ofcoating (Cr content): 20 mg/m².

Inorganic chromate 2{circle around (1)}: Comparative Example I solution,the amount of coating (Cr content): 150 mg/m².

Inorganic chromate 2{circle around (2)}: Comparative Example I solution,the amount of coating (Cr content): 150 mg/m².

Composite chromate 1: Example C solution, the amount of coating (Crcontent): 30 mg/m².

Composite chromate 2: Example E solution, the amount of coating (Crcontent): 120 mg/m².

With this type of resin-rich chromate film, it is possible to accomplishthe treatment with one less step compared to resin coating treatmentafter chromate treatment, which is the standard organic coatingtreatment, and it is therefore a more advantageous treatment in terms ofcost. In addition, by using a low temperature curable resin, there is afurther advantage in that no special dry furnace is necessary andtreatment is possible with conventional chromate treatment equipment.

Changes in the composition of this composite chromate film which hasexcellent weldability in terms of the resin/chromium weight ratio aftercuring will alter the performance of the film. For example, a lowresin/chromium ratio (weight ratio) will tend to result in poorerweldability since the proper contact resistance value will not beobtained. On the other hand, a larger resin/chromium ratio will reducethe corrosion resistance and somewhat impair the weldability.Consequently, the post-curing weight ratio value for the resin/chromiumratio of the composite chromate film for this purpose is preferred to beabout 0.5-18.

The chromium or chromic acid compound used according to the inventionmay be either or both chromic anhydride or a reduced aqueous chromicacid solution with an adjusted Cr³⁺/Cr⁶⁺ compositional ratio by reactionof an aqueous chromic acid solution with a reducing agent. When reducedchromic acid is used, the reducing agent used may be starch, asaccharide, alcohol or other organic compound, or hydrazine,hydrophosphorus acid or another inorganic compound.

Suitable resins which may be used according to the invention includewater-soluble organic polymer compounds, specificallycarboxyl-containing anionic polyacrylic acid and polymethacrylic acidand their copolymer compounds, maleic acid copolymer compounds, vinylacetate copolymer compounds, vinyl carboxylate ester, vinyl ether,styrene, acrylamide, acrylonitrile, vinyl halides and other ethylenicunsaturated compounds, polyethylene compounds, polyurethane compounds,epoxy resin compounds, polyester compounds, etc. These organic polymercompounds are mainly added alone when used, but two or more types mayalso be added in combination. Among them, emulsion-type resins areparticularly preferred when conventional chromate equipment is usedbecause they are suitable for low temperature baking. Also, addition ofa small amount of a lubricant or antirust pigment to the resin is notcontrary to the gist of the invention.

According to the invention, the composite chromate treatment is carriedout by a step following plating. The treatment is primarily for thepurpose of weldability, but since resin chromates have lubricity, theyalso have the advantage of improved workability. While this is thereason for limitation to a composite chromate, the composite chromatemay also contain added silica for the purpose of improving corrosionresistance and phosphoric acid for the purpose of reducing theyellowness of the chromate.

The thickness of the composite chromate film is restricted to 0.1-2 μm.At less than 0.1 μm it is impossible to form a film which is sound interms of the resin, and with a film of greater than 2.0 μm theresistance value is too high, impeding electric conduction between theelectrode and the steel sheet or between steel sheet and steel sheet,thus making welding itself impossible. The composite chromate treatmentmay involve coating on either or both sides, but the ideal filmthickness is slightly different depending on whether it is on one orboth sides. Since the heat release during welding generally depends onthe contact resistance between the adjacent steel sheets, compositechromate treatment of over 1.0 μm on both sides will produce a resinchromate film of over 2.0 μm between the steel sheets, thereby impedingelectric conduction between the steel sheets. When coating both sides,therefore, it is preferred for each film to be 1.0 μm or less, and inthe case of one sided coating, it is preferred for the resin sides to bedesignated as inside and outside when combined.

For improved uniform coatability of the treatment solution and improvedcorrosion resistance and coating performance of the chromate film, thechromate treatment solution of the invention may also contain aphosphoric acid compound and/or colloidal silica comprising either orboth silica and a silicate. The phosphoric acid compound is added in arange of 100 parts by weight to 600 parts by weight to 100 parts byweight of Cr in the chromic acid. At less than 100 parts by weight theeffect of its addition will be insufficient, and at greater than 600parts by weight the chromate film will tend to absorb water, thusimpairing the corrosion resistance. The colloidal silica comprisingeither or both silica and a silicate is in the range of 100 parts byweight to 1000 parts by weight to 100 parts by weight of Cr in thechromic acid. At less than 100 parts by weight the uniform coatabilitywill be impaired, making it difficult to ensure corrosion resistance andcoating performance, while at greater than 1000 parts by weight theeffect will be saturated.

In order to form a chromate film with more excellent corrosionresistance and coating adhesion, phosphonic acid or a phosphonic acidsalt compound may also be added to the inorganic and organic compositechromate film of the invention. The phosphonic acid is preferably addedat 10 parts by weight to 200 parts by weight to 100 parts by weight ofCr in the chromic acid. If the phosphonic acid is added at less than 10parts by weight, there will be a reduced surface cleansing effect byetching of the phosphonic acid and reduced anticorrosion and coatingadhesion effects by uniform formation of the film and by its inclusionin the film. The phosphonic acid is preferably not added at greater than200 parts by weight because the effect of its addition will be saturatedand the stability of the treatment bath will be lower.

When the composite chromate film is applied on only one side, moresatisfactory corrosion resistance can be ensured by forming on the otherside a chromate film containing silica, preferably. The chromate filmmay be formed by a conventional publicly known method, with a amount ofcoating of from 10 mg/m² to 200 mg/m². At less than 10 mg/m²satisfactory corrosion resistance cannot be sufficiently obtained forfuel tanks, and at greater than 200 mg/m² the effect will be saturated.

According to the invention the chromate treatment is carried out in astep following the plating, and the manufacturing process may beapplication, immersion, spraying or any other publicly known process.

According to one preferred mode of the invention for carrying out thefirst embodiment which employs an organic and inorganic compositechromate film, not only the resistance weldability and corrosionresistance but also the continuous press formability can be improved byadding a prescribed amount of a lubricant to the composite chromatefilm. Also, since it is possible to accomplish the treatment with oneless step compared to resin coating treatment after inorganic chromatetreatment, which is the standard organic coating treatment, it istherefore a superior treatment in economic terms. In addition, by usinga low temperature curable resin, there is a further advantage in that nospecial dry furnace is necessary and treatment is possible withconventional chromate treatment equipment.

Specifically, a composite chromate film to which a lubricant is added at0.5-20 wt % is formed to a thickness of 0.1-2 μm on the aluminizinglayer(s) of one or both sides of the aluminized steel sheet. Instead offorming the composite chromate film on both sides of the plated steelsheet, an organic and inorganic composite chromate film may be formed onone side and an inorganic-based chromate film, organic film or anorganic film on an inorganic-based chromate film may be formed on theother side.

The lubricant added for improved press formability is preferably onewhich disperses and dissolves easily in water, since the resin is anaqueous system. Such lubricants include ester-based, brazingmaterial-based, stearic acid-based, silicon-based special olefin-basedand paraffin brazing material-based lubricants. Based on the experienceof the present inventors all such lubricants exhibit their correspondingeffects, but stearic acid-based lubricants have been most effective.

FIG. 3 is a graph showing the changes in Bowden friction coefficients offilms containing different amounts of stearic acid-based lubricants(measuring conditions: 10 mm diameter steel ball, 500 g load, averagevalue n=5) and the blackened state of tape applied to the external sidesof samples after cylindrical drawing with a diameter of 70 mm and adepth of 40 mm (see examples for evaluation scale, etc.). An effect wasfound from a content of 0.5 wt %, and workability improved as thelubricant content increased. At over 20 wt %, however, the effect tendedto become saturated, while dispersion and dissolution in the compositechromate solution was hindered so that gelation of the solutionoccurred. Hence, the lubricant is added at 0.5-20 wt %, and preferably0.5-15 wt %.

Treatment on the other side of the single-side compositechromate-treated material may be appropriately selected (the other sidemay be left untreated) depending on the need. That is, the inorganicchromate film may be formed at sections which do not require strictformability, such as on separator and subtank members used inside thetank. Sections which require lubrication and weldability, such as theexterior of the tank, may be subjected to organic film treatment ororganic film treatment on inorganic chromate. Since the tank exterior isgiven a thick coating in the final step, less corrosion resistance isrequired for the thin-film on the plating. However, it does requirecoating adhesion, and organic film treatment on inorganic chromate willbe more stable than a simple organic film layer. The “inorganicchromate” referred to here may be a coating type, reaction type orelectrolytic type. The aforementioned lubricant may also be added to theorganic film.

Another preferred mode of the invention, however, is to form the organicand inorganic composite chromate film on one side of a steel sheet whichis aluminized on both sides, and to form an inorganic-based chromatefilm C on the other side to 200 mg/m² or less in terms of Cr, or form aninorganic-based chromate film C to 100 mg/m² or less in terms of Crbetween the composite chromate film and the aluminizing layer. Thisinorganic-based chromate film C preferably contains a small amount (lessthan 50 wt %) of either or both an organic phosphoric acid andphosphonic acid or a phosphonic acid salt compound.

As a result of much research intended to improve the resistanceweldability of aluminized steel sheets, the present inventors have foundthat the weldability can be vastly improved by coating the surface of analuminized steel sheet with an oxide film, chromate film, organic resinfilm or the like. It was found that this effect increases the contactresistance between steel sheets due to the film, thus acceleratingformation of welding nuggets by providing adequate heat between thesteel sheets even under a low welding current, while also suppressingreaction between the welding electrode chips and plating metal becauseof the film, so that the life of the electrode can be extended.

Materials wherein an organic resin film is coated on both sides of thealuminized steel sheet have both sides coated with an organic resin, andtherefore the treatment cost is greater than by conventionalinorganic-based chromate treatment, and corresponding treatmentequipment (roll coater, electrostatic coating apparatus, etc.) must beprovided for both sides. It also requires a dry furnace which allowscuring under relatively high temperatures.

This mode of the invention was developed with the goal of achievingsuitable treatment cost and weldability. That is, a composite chromatefilm comprising a resin and a chromic acid compound is formed to anappropriate thickness on one side of the steel sheet, while on the otherside there is formed an inorganic-based chromate film comprising achromic acid compound and silica or an inorganic-based chromate filmcontaining either or both an organic phosphoric acid and a small amountof a resin, or optionally an inorganic chromate film or aninorganic-based chromate film containing either or both an organicphosphoric acid and a small amount of a resin is formed between thecomposite chromate film and the plating layer. Development of thistreatment was completed after it was found to exhibit corrosionresistance and other effects at a relatively lower cost than spotwelding, seam welding and other types of common resistance welding.

The composite chromate film exhibits sufficient corrosion resistanceunder normal conditions, but for even greater corrosion resistance,inorganic-based chromate treatment may be carried out at the interfacebetween the composite chromate film and the plating layer. For example,in cases where deep working defects have been generated in thealuminizing layer, since elution of chromic acid in the film isinhibited by the resin, less rustproofness is often exhibited ascompared to inorganic-based chromates, and depending on the environmentrust may tend to be generated from defect locations. It was found thatby accomplishing this treatment it is possible to further improve theanticorrosion performance in addition to giving the satisfactoryresistance weldability described above.

In this case, the amount of coating of the inorganic-based chromate filmshould be 100 mg/m² or less in terms of metallic chromium. At greaterthan 100 mg/m² the effect of corrosion resistance will be saturated,while the thickness of the film including that of the composite chromatefilm will increase, thus raising the contact resistance value andadversely affecting the weldability.

While the composition of the inorganic-based chromate film is notparticularly restricted, it may be a chromic acid compound/silicamixture solution, and one or more from among phosphoric acid, organicphosphoric acids such as phosphonic acid or phosphonic acid saltcompounds and resins may also be added. However, if the organicphosphoric acid or resin is added in too great an amount the cost burdenwill increase, and the effect (corrosion resistance improvement, etc.)will become saturated. The concentration ratio of the organic phosphoricacid/chromic acid compound may be ≦1, and the concentration ratio of theresin/chromic acid compound may be ≦1.

(Second embodiment)

According to a second embodiment of the coating aluminized steel sheetof the invention, an inorganic-based chromate film A is formed on thealuminizing layer on one or both sides of the aluminized steel sheet,and specifically an inorganic-based chromate film is formed whichcomprises 100 parts by weight of the chromic acid compound in terms ofmetallic chromium and 100-1000 parts by weight of colloidal silica, andfurther comprises at least one selected from among 100-600 parts byweight of a phosphoric acid compound, 10-200 parts by weight of aphosphonic acid or phosphonic acid salt compound and less than 50 partsby weight of an organic resin.

The inorganic-based chromate is a type whose main components are achromic acid compound and colloidal silica and which contains phosphoricacid, a phosphonic acid or phosphonic acid salt compound or a smallamount of a resin, and it can be employed sufficiently in practical usedespite its slightly poorer weldability than materials having both sidescomposite chromate-treated, because the treatment can be accomplished atlower cost compared to resin applications and composite chromates; theyalso provide some degree of corrosion resistance, and there is an effectwhich increases the contact resistance value between steel sheets andinhibits reaction between the welding electrode and plating metal.

For the inorganic-based chromate film A according to this embodiment,the chromium or chromic acid compound, phosphoric acid compound,colloidal silica, phosphonic acid or phosphonic acid salt compound andresin are the same as used for the first embodiment.

The chromate treatment solution for this embodiment may also contain aphosphoric acid compound and/or colloidal silica comprising either orboth silica and a silicate, for more uniform application of thetreatment solution and improved corrosion resistance and coatingperformance for the chromate film. The phosphoric acid compound is addedin the range of 100 parts by weight to 600 parts by weight to 100 partsby weight of Cr in the chromic acid. At less than 100 parts by weightthe effect of addition will be insufficient, and at greater than 600parts by weight the chromate film will tend to absorb water, thusimpairing the corrosion resistance. The colloidal silica comprisingeither or both silica and a silicate is added in the range of 100 partsby weight to 1000 parts by weight to 100 parts by weight of Cr in thechromic acid. At less than 100 parts by weight the uniform coatabilitywill be impaired, making it difficult to ensure corrosion resistance andcoating performance, while at greater than 1000 parts by weight theeffect will be saturated.

In order to form a chromate film with more excellent corrosionresistance and coating adhesion, phosphonic acid or a phosphonic acidsalt compound may also be added to the inorganic-based chromate film ofthe invention. The phosphonic acid is preferably added at 10 parts byweight to 200 parts by weight to 100 parts by weight of Cr in thechromic acid. If the phosphonic acid is added at less than 10 parts byweight, there will be a reduced surface cleansing effect by etching ofthe phosphonic acid and reduced anticorrosion and coating adhesioneffects by uniform formation of the film and by its inclusion in thefilm. The phosphonic acid is preferably not added at greater than 200parts by weight because the effect of its addition will be saturated andthe stability of the treatment bath will be lower.

The thickness of the inorganic-based chromate treatment film A is 200mg/m² or less in terms of metallic chromium. Satisfactory resistanceweldability can be achieved within this range, but more satisfactoryresistance weldability is achieved between 75 mg/m² and 120 mg/m². Ifthe amount of coating exceeds 200 mg/m², the insulating property willincrease, thus impairing the weldability. Conversely, if it is too lowthe effect of inhibiting reaction between the electrode and plating willbe unstable, and the weldability will tend to be poorer. A chromiumamount of coating of 10-200 mg/m² is preferred.

(Third embodiment)

According to a third embodiment of the coating aluminized steel sheet ofthe invention, an inorganic-based chromate film B is formed on thealuminizing layer on one or both sides of the aluminized steel sheet.

Here, the inorganic-based chromate film B is an inorganic-based filmcomposed mainly of a conventional known chromium (chromic anhydride),and if necessary including admixture of silica or other additives.

As a result of much research on aluminized steel sheets with excellentcorrosion resistance, formability and weldability, the present inventorshave achieved development of a steel sheet with excellent properties forfuel tanks by treatment of the surface with a suitable amount of aninorganic-based chromate.

As a result of further research on properties required for fuel tanksand the details involved in their manufacture, the present inventorsfound that it is advantageous to form the inorganic-based chromate filmon the surface of the Al-based plating layer to at least 10 mg/m² andless than 35 mg/m², and for use as a fuel tank material, preferably atleast 20 mg/m² and less than 30 mg/m². From the standpoint of corrosionresistance, at less than 10 mg/m² the effect is insufficient and thereare concerns of corrosion from plating layer cracks at worked sections.The plating metal also tends to adhere to the electrode during spotwelding, thus hindering continuous operation. An amount of coating of 10mg/m² or greater gives sufficient corrosion resistance and resistanceweldability for fuel tanks, but at 20 mg/m² or greater the resistanceweldability is even more satisfactory. If the amount of coating is 35mg/m² or greater, however, the corrosion resistance is satisfactory butproblems result in terms of weldability, such as reduced brazingmaterial wettability with certain brazing material materials.

With these considerations, therefore, a lower Cr amount of coating ispreferred for brazing properties, and the present inventors establishedan upper limit of less than 35 mg/m², and preferably no greater than 30mg/m² for fuel tanks.

According to the invention, the inorganic-based chromate treatment iscarried out in a step following plating, but there are no particularrestrictions on the composition of the inorganic-based chromatetreatment solution. The composition of the inorganic-based chromate filmmay be that of an inorganic-based chromate treatment solution with apublicly known composition, and the production process may be anypublicly known process, such as immersion, spraying, electrolysis,application or the like.

For this embodiment, the aluminizing layer is suitably Al or an Al alloywith 3-15% Si.

Automobile Fuel Tank

According to the invention there is provided a fuel tank, especially anautomobile fuel tank, produced using the aforementioned coatingaluminized steel sheet. The fuel tank contains no Pb in light ofenvironmental considerations and has the above-mentioned excellentproperties of corrosion resistance, press formability and weldability,and it is particularly useful as an automobile fuel tank, such as anautomobile gasoline tank, alcohol fuel tank, etc.

According to one aspect of the invention, it is an automobile fuel tankwherein a pair of bowl-shaped bodies with flanges are integrated bycontinuous seam-welding of the flange substances, the automobile fueltank being characterized in that the materials of which the bowl-shapedbodies are made are coating aluminized steel sheets which consist ofaluminized steel sheets each having on one or both sides an aluminizinglayer based on aluminum or an aluminum alloy containing 2-13 wt %silicon, and having a resin coating on the uppermost surface of theinner and/or outer side.

Specifically, it was found that working of steel sheets produces cracksin the aluminizing layer because of the poor lubricity of the aluminizedplating surface, and in order to prevent this the aluminizing surfacewas provided with a satisfactorily lubricous resin film which vastlyimproved the corrosion resistance after working.

The automobile fuel tank is formed by forming upper and lower tankmembers into bowl shapes with flanges by pressing or the like, andcombining the upper and lower members and seam welding the flangesections. This structure is not particularly limited, but it ispreferably equipped with a fuel supply opening, a fuel supply pump, afuel hose, a fuel hose which returns excess fuel, separators to preventthe sound of fuel waves, etc., as in a normal fuel tank.

FIG. 4 is a cross-sectional illustration of the lower part of anautomobile fuel tank. This tank is an example wherein resin films 12, 13are formed on the uppermost surfaces of both sides of an aluminizedsteel sheet 11. The resin film 12 can provide a lubricating function,especially on the inside, when forming is accomplished by deep drawingwhile press forming.

The method for bonding the members may be spot welding, soldering orbrazing. The difference between soldering and brazing is not clearlydefined, but in this specification brazing will be considered weldingwith a metal having a melting point of 450° C. or higher, and solderingthe use of a metal with a melting point below that temperature. Themajor feature of this fuel tank is the material of which the fuel tankis composed, i.e. not only the tank body but also the internalseparators, supply openings, etc. are made of materials containingsubstantially no Pb. The conventional fuel tank body containing Pb isreplaced with an aluminized steel sheet having a resin film on theuppermost surface. The soldering and brazing materials may also bealuminum-based materials containing substantially no Pb.

With conventional naked aluminized steel sheets there has been a concernof drastic reduction in corrosion resistance by working, but accordingto the invention this is solved with a resin film on the uppermostsurface layer. This is connected with the fact that naked aluminizedsteel sheets have poor lubricity even when oiled, and cracks areproduced in the platings, drastically impairing the corrosionresistance; however, formation of a highly lubricous resin film on thesurface succeeded in suppressing cracks in the plating. If the filmthickness is too low the film will not cover the entire surface, and theeffect of improved corrosion resistance after press forming will belessened. A higher film thickness is advantageous for corrosionresistance after press forming, but if it is too high the welding,soldering or brazing becomes more difficult, thus hindering theproduction efficiency of the fuel tanks.

The thickness of the resin film according to this aspect of theinvention is preferably 0.1-2 μm after forming. It is more preferably0.3-1 μm. The resin film provides its effect whether it is formed onboth sides, on the outer side alone or on the inner side alone. Whilethe effect can be easily imagined if the film is on the inner side, itis believed that there is an effect even on the outer side alone, forthe following reason. Common press forming is used for forming of thefuel tank, and the surface lubricity is a major factor contributing tothe press formability. The lubricity of the outer side is a particularlyimportant factor here, and therefore even if the film is only on theouter side it is thought to have an effect on the inner side as well inthe sense of preventing damage to the plating.

The material for brazing or soldering of the fuel tank may also be, forexample, aluminum-based. Soldering or brazing of an aluminum surface isusually considered to be difficult because of the stable passive film onthe aluminum surface, but highly productive joints can be achieved byusing appropriate flax. An aluminum-based brazing material has a highermelting point than conventional Pb—Sn-based solder, and thereforesatisfactory brazing can be accomplished even with a resin film.Ni-based materials can also be used.

The fuel tank surface has a resin film, but no particular restrictionsare placed on the composition and structure of the resin film. Examplesof suitable systems which may be used for the resin includewater-soluble organic polymer compounds, specificallycarboxyl-containing anionic polyacrylic acid and polymethacrylic acidand their copolymer compounds, maleic acid copolymer compounds, vinylacetate copolymer compounds, vinyl carboxylate ester, vinyl ether,styrene, acrylamide, acrylonitrile, vinyl halides and other ethylenicunsaturated compounds, polyethylene compounds, polyurethane compounds,epoxy resin compounds, polyester compounds, etc. These organic polymercompounds are mainly added alone when used, but two or more types mayalso be added in combination. However, the resin film used isparticularly preferred to be a resin/inorganic composite chromate film.The composite chromate is prepared by mixing chromic acid with the resinin a chromate treatment solution, for even dispersion of the chromicacid compound in the resulting film. The Cr⁶⁺ contained therein elutesduring use of the tank, to give stabilized corrosion resistance. It isespecially preferred to use a coating aluminized steel sheet accordingto the invention having one of the types of chromate films describedabove (organic and inorganic composite chromate film, inorganic-basedchromate film A, inorganic-based chromate film B).

This treatment is also more advantageous in terms of cost as compared tostandard resin film treatment involving resin coating after chromatetreatment, since the treatment can be accomplished in a single step. Inaddition, by using a low-temperature curable resin, there is a furtheradvantage in that no special dry furnace is necessary and treatment ispossible with conventional chromate treatment equipment. Whenconventional chromate equipment is used, the type of resin used ispreferably an emulsion type which can be baked at low temperatures.Furthermore, addition of a small amount of a lubricant, antirust pigmentor the like to the resin can also enhance the effect.

Welding Process for Fuel Tank

As a result of much research on surface treatment and welding processesfor aluminized steel sheets with excellent resistance weldability andcontinuous operation suitability, the present inventors have found thatthe problems of welding described above can be solved and vastimprovement in continuous operation can be achieved by forming a resincoating layer or a chromate-containing resin coating layer on one orboth sides of aluminum-based plated steel sheets, and welding the steelsheets by an appropriate method of combination.

Specifically, two resin-coated aluminum-based plated steel sheets, eachof which has a plating layer comprising aluminum and unavoidableimpurities or comprising 2-13 wt % Si and the remainder aluminum andunavoidable impurities formed on one or both sides, as well as a resincoating layer provided on the one or both sides, are combined and seamwelded between a pair of electrode wheels, wherein at least the sidescorresponding to the inner side of the fuel tank have an aluminum-basedplating layer, and a resin coating layer is provided on at least one ofthe steel sheet surfaces at the side where the steel sheets meet and/oron at least one of the steel sheet surfaces at the side where itcontacts with the electrode wheel.

As explained above, the steel sheet-coating aluminum reacts readily withthe electrode Cu, resulting in the problem of more rapid electrode lossand poorer continuous operation. Accordingly, there are two importantobjects for improved continuous operation: minimizing the electrode lossand increasing the contact resistance value between the steel sheets inorder to form more efficient nuggets. The present inventors havediscovered that for this purpose, formation of a resin coating layer onone or both sides of each aluminized steel sheet and welding of thesteel sheets by a suitable method for combination can effectively ensuresatisfactory resistance weldability and improve continuous operation,and the present invention has thus been completed.

FIGS. 5A-5C are illustrations of seam welding of automobile fuel tanks.FIG. 5A is a perspective view, FIG. 5B is a lower view of FIG. 5A, andFIG. 5C is a cross-sectional view. In these illustrations, the upper andlower tank members 21, 22 formed by deep drawing of steel sheets arecontacted together toward the inside of the tank at the flange sections23, 24 with the exterior sides of the tank sandwiched between electrodewheels 25, 26 for seam welding, and a current flows between theelectrode wheels 25, 26 to weld the flange sections (seam weldingsection 27) while the fuel tank is rotated (in direction A) so that theentire flange section perimeter is welded (direction B).

In this type of seam welding, the presence of the resin film sidebetween the steel sheets has increased the contact resistance valuebetween the steel sheets, as shown in FIG. 6A. Consequently, bysituating the resin coating side between the steel sheets the improvedcontact resistance value between the steel sheets can providesatisfactory nugget formation due to accelerated heating. In addition,when a resin film side is present between the steel sheet and electrode,the resistance value is virtually the same as with an untreated materialeven though one layer of film is present between them. Thus, situating aresin coating side between the steel sheet and electrode can provide aneffect of lower electrode loss due to the protecting action of the film.This is attributed to the fact that a more uniform thin layer ispossible during pressurization since the resin is soft and forms a toughfilm, so that uniform weld current passing points are produced. Thisfunction provides an effect whether the resin film is present on atleast one of the steel sheet surfaces between the combined steel sheets,or whether the resin film is present on the side of the steel sheetcontacting the electrode wheel. The effects are cumulative when thetreatment is on both sides, so that the overall effect is greater.

FIG. 6A is a bar graph showing the contact resistance values betweenupper electrode and sheet, the contact resistance values between sheetand sheet and the contact resistance values between sheet and lowerelectrode for the different sample steel sheets described below andillustrated in FIGS. 6B-6D. In FIGS. 6B-6D, 31 and 32 are flangesections of aluminized steel sheets, 31 a and 32 a are resin films onthe sides between steel sheets (inner sides), and 31 b and 32 b areresin films on the sides of the electrode wheels 35, 36 (outer sides).

Resin coating material {circle around (1)}: Combination of both-sideresin coated materials (1 μm epoxy resin)

Resin coating material {circle around (2)}: Combination of one-sidecoated material (1 μm epoxy resin, steel sheet side) and one-side coatedmaterial (1 μm epoxy resin, electrode side)

Resin coating material {circle around (3)}: Combination of one-sidecoated material (1 μm epoxy resin, steel sheet side) and untreatedmaterial

The resin coating amount which expresses the effect described above isat a thickness of 0.1-2 μm. At less than 0.1 μm its contribution toresistance weldability is insufficient, and at greater than 2 μm thetotal thickness between steel sheets when both sides are treated is over4 μm, resulting in an excessively large contact resistance value andpoor continuity.

The resin used for the invention may be either water-soluble or asolvent system. Examples include water-soluble organic polymercompounds, specifically carboxyl-containing anionic polyacrylic acid andpolymethacrylic acid and their copolymer compounds, maleic acidcopolymer compounds, vinyl acetate copolymer compounds, vinylcarboxylate ester, vinyl ether, styrene, acrylamide, acrylonitrile,vinyl halides and other ethylenic unsaturated compounds, polyethylenecompounds, polyurethane compounds, epoxy resin compounds, polyestercompounds, etc. These organic polymer compounds are mainly added alonewhen used, but two or more types may also be added in combination.

Also, while an adequate effect is exhibited with a resin film alone,satisfactory resistance weldability can also be obtained withsatisfactory corrosion resistance by application of a treatment solutionin combination with a chromate treatment solution composed mainly ofchromic acid, to form an organic and inorganic composite chromate filmor the above-mentioned inorganic-based chromate film A (resin-added),especially when the resin system is water-soluble.

The resin/chromate combination treatment solution may also containsilica or phosphoric acid for enhanced corrosion resistance, coatingadhesion and uniform coatability.

According to the invention, the resin coating layer is formed in a stepfollowing plating, and the production process may be any publicly knownprocess, such as application, immersion, spraying or the like.

The aluminum plating with formation of a resin coating layer may be andis preferred to be as described above.

EXAMPLES

In the examples which follow, the following performance evaluationmethods were employed.

(1) Press formability evaluation

{circle around (1)} Cylindrical drawing test A

A forming test was carried out with a hydraulic forming tester using a50-mm diameter cylindrical punch at a draft of 2.3. The blank holdingpressure was 500 kg, and the formability was evaluated according to thefollowing scale.

⊚: Formable, no plating layer defects

∘: Formable, slight damage to plating layer

Δ: Formable, peeling of plating layer

x: Unformable

{circle around (2)} Cylindrical drawing test B

A forming test was carried out with a hydraulic forming tester using a70-mm diameter cylindrical punch at a draft of 2.3. The blank holdingpressure was 1000 kg, and the formability was evaluated based on outerappearance of the shaped cylinder and visual judgment of blackening ofapplied tape.

(Evaluation scale)

⊚: Formable, no plating layer defects, no blackening of tape

∘: Formable, no plating layer defects but slight blackening of tape

Δ: Formable, some flaws in plating layer, blackening of tape

x: Unformable, peeling of plating layer

{circle around (3)} Bowden friction coefficient measurement

Measured by the Bowden method using a 10 mmφ stainless steel sphere witha load of 500 g. The measurement included scanning 10 times at the samelocation, and the average value was determined.

(Evaluation scale)

⊚: Friction coefficient ≦0.1

∘: 0.1< friction coefficient ≦0.25

Δ: 0.25< friction coefficient ≦0.4

x: 0.4< friction coefficient

(2) Weldability evaluation

{circle around (1)} Spot welding

Spot welding was carried out under the welding conditions describedbelow, and the number of continuous weld points to the time at which thenugget diameter cleared 4{square root over (t)} (t=sheet thickness) wasevaluated. For one-sided coatings, the evaluation was made with theresin side on the inside and outside when the sheets were combined.

(Welding conditions)

Welding current: 10 kA, pressure force: 220 kg, welding time: 12 cycles,electrode tip diameter: 6 mmφ, electrode shape: dome

(Evaluation scale)

⊚ex (excellent): 1500 or more continuous weld points

⊚ (very good): 1000-less than 1500 continuous weld points

∘ (good): 500-less than 1000 continuous weld points

Δ (fair): 250-less than 500 continuous weld points

x (not good): less than 250 continuous weld points

{circle around (2)} Seam weldability evaluation

An R6 mm-φ250 m electrode wheel was used for 10 m of seam welding at awelding current of 13 kA, a pressure force of 400 kg and anelectrization of 2 on-2 off, after which a test sample was preparedaccording to JIS-Z-3141 and subjected to a leaking test. Evaluation Awas made on the following scale.

∘: No leaking

x: Leaking

Simultaneously with the leaking test, the cross-section weld penetrationand contamination of the electrode surface were observed for evaluationB on the following scale.

⊚: No leaking (satisfactory weld penetration, virtually no contaminationof electrode surface)

∘: No leaking (satisfactory weld penetration, little contamination ofelectrode surface)

Δ: No leaking (satisfactory weld penetration, much contamination ofelectrode surface)

x: Leaking (abundant opened holes or poor weld penetration, muchcontamination of electrode surface)

{circle around (3)} Brazing evaluation

The brazing material spread was evaluated according to JIS Z-3191. Aflat sample was toluene-degreased and then flax was coated onto thesheet, a fixed amount of brazing material was applied, the sample washeated at a prescribed temperature for a given time in an heatingfurnace, and the area of brazing material spread was measured.

(Test conditions)

Brazing material: Al-10% Si brazing material (100 mg), flax:chloride/fluoride system (AWS Nol), heating temperature: 590° C.,heating time: 30 sec.

(Evaluation)

⊚: Satisfactory spreading

∘: Satisfactory spreading but slight edge sinking

Δ: Some spreading with edge sinking and caving

x: Almost no spreading

(3) Corrosion resistance evaluation

{circle around (1)} Plated steel sheet test

The corrosion resistance against gasoline was evaluated. In the methodemployed, a test fluid was placed in a sample with a 20 mm flange, 50 mmdiameter and 25 mm depth which had been worked by flat-bottomcylindrical drawing with a hydraulic forming tester, and the sample wascovered with glass via a silicon rubber ring. The condition of corrosionafter the test was visually observed. Those materials treated on onlyone side were tested on their treated side.

(Test conditions)

Test fluid: gasoline+10% distilled water+200 ppm formic acid

Test period: 3 months at 40° C.

(Evaluation scale)

⊚: No change

∘: White rust of 0.1% or less

Δ: Red rust of 5% or less, or white rust of 0.1%-50%

x: Red rust of over 5% and considerable white rust

{circle around (2)} Fuel tank test

The corrosion resistance against gasoline was valuated. In the methodemployed, a shaped fuel tank was kept at constant temperature while atest fluid was continuously circulated therein. After the test, thecondition of corrosion of the cut fuel tank was visually observed.

(Test conditions)

Test fluid: gasoline+10% distilled water+200 ppm formic acid

Test period: 3 months at 40° C.

(Evaluation scale)

∘: Red rust of less than 0.1%

Δ: Red rust of 0.1-5%, or white rust present

x: Red rust of over 5% and considerable white rust

{circle around (3)} Pb elution

After the above test (3), the amount of Pb eluted into the test fluidwas quantified by a wet method and used to evaluate the Pb elution.

(Evaluation scale)

∘: No elution (below detection level)

x: Elution

{circle around (4)} Flaw corrosion

A cross-cut flaw was made in a 70 mm×150 mm piece, and the rustgeneration was determined by a salt spray test. Both the resin chromatetreated side and inorganic-based chromate side were evaluated.

(Test conditions)

Salt spray test: Rate of rust generation after 240 hours

(Evaluation scale)

(∘ex: no rust generation)

∘: less than 5% white rust

Δ: 5-50% white rust or less than 5% red rust

x: over 50% white rust or considerable red rust

Examples 1-28

Steels having the components listed in Table 1 were prepared as ingotsby conversion/vacuum degassing processing, and steel samples weresubjected to hot rolling and cold rolling under normal conditions toobtain cold-rolled steel sheets (thickness: 0.8 mm).

TABLE 1 Plating sheet components (wt %) Sample C Si Mn P S Ti Al B N A0.0012 0.03 0.32 0.007 0.009 0.054 0.04 0.0003 0.0033 B 0.0020 0.09 0.320.008 0.011 0.040 0.04 — 0.0032

These materials were used for hot dip aluminizing. The hot dipaluminizing was accomplished using a non-oxidizing furnace/reducingfurnace type line, and annealing was also carried out in this fusedplating line. The annealing temperature was 800-850° C. After plating,the amount of plating was adjusted by the gas wiping method. Here, theplating temperature was 660° C., the plating bath composition wasbasically Al-2% Fe, and Si was also added. The Fe in the bath wassupplied from plating equipment and strips in the bath.

Aluminized steel sheets produced in this manner were subjected tocomposite chromate treatment with the bath of Table 2 as the standardcomposition. Baths with the same (resin amount+chromic acid amount) inTable 2 but with different resins/chromic acid were also used. The filmthickness was adjusted with a linger roll, and hot air at 80° C. wasused for drying to complete the film.

TABLE 2 Standard composition of composite chromate treatment solution(g/l: in terms of pure composition) Concentration Resin 120 Chromic acid30 Phosphoric acid 60 Colloidal silica 10

The performance of steel sheets produced in this manner as fuel tankswas evaluated. The evaluation method used here was the following, andthe plating conditions and performance evaluation results are shown inTables 3 and 4.

Press formability: Cylindrical drawing test A

Weldability: Spot weldability evaluation

Corrosion resistance: Plated steel sheet test

As shown in Table 4, when the Si content of the plating is too low(Comparative Example 23) the alloy layer grows too much, resulting inpeeling of the plating during working. Conversely, when the Si contentis too high (Comparative Example 24), the corrosion resistance isimpaired. When the amount of aluminizing layer is too great (ComparativeExample 25) the welding section is inferior. When the film thickness istoo small (Comparative Example 26) or too large (Comparative Examples27, 28), satisfactory weldability cannot be obtained. By manufacturingthe platings with the satisfactory plating composition, the amount ofplating and composite chromate conditions, hot dip aluminized steelsheets with excellent press formability, weldability, outer appearanceand corrosion resistance can be obtained. However, when theresin/chromium ratio is low or high (Examples 19, 22) the weldability isslightly impaired, and therefore the resin/chromium ratio is preferredto be a proper value.

Examples 1-23 provide hot dip aluminized steel sheets which have boththe corrosion resistance and press formability required for automobilefuel tank materials, as well as achieving the weldability which has beena problem in the past, and they are therefore very promising as new fueltank materials and represent a major contribution to industry, as asolution to future difficulties involved with using Pb-based materialswhich have become an environmental problem.

TABLE 3 Composite Si Amount of chromate film Resin/ content platingthickness (μm) Major resin chrom- Ex. in bath of one side (one or eachof composite ium no. Sheet (wt %) (g/m²) of both sides) chromater filmratio Inven- 1 A 9.4 30 both: 0.4 acryl. acid ester 8.0 tion 2 B 9.4 30both: 0.4 acryl. acid ester 8.0 Exs. 3 A 5.2 30 both: 0.4 acryl. acidester 8.0 4 A 11.4  30 both: 0.4 acryl. acid ester 8.0 5 A 9.4 30 both:0.4 acryl. acid ester 8.0 6 A 9.4 30 both: 0.2 acryl. acid ester 8.0 7 A9.4 30 both: 0.8 acryl. acid ester 8.0 8 A 9.4 30 both: 1.2 acryl. acidester 8.0 9 A 9.4 30 one: 0.2 acryl. acid ester 8.0 10 A 9.4 30 one: 0.4acryl. acid ester 8.0 11 A 9.4 30 one: 0.8 acryl. acid ester 8.0 12 A9.4 30 one: 1.2 acryl. acid ester 8.0 13 A 9.4 30 one: 1.8 acryl. acidester 8.0 14 A 9.4 30 both: 0.4 vinyl carboxy- 8.0 late ester 15 A 9.430 both: 0.4 vinyl ether 8.0 16 A 9.4 30 both: 0.4 styrene 8.0 17 A 9.430 both: 0.4 acrylamide 8.0 18 A 14.5  30 both: 0.4 epoxy 8.0 19 A 9.430 both: 0.4 acryl. acid ester 1.0 20 A 9.4 30 both: 0.4 acryl. acidester 4.0 21 A 9.4 30 both: 0.4 acryl. acid ester 12.0 22 A 9.4 30 both:0.4 acryl. acid ester 18.0 Comp. 23 A 1.5 30 both: 0.4 acryl. acid ester8.0 Exs 24 A 16.0  30 both: 0.4 acryl. acid ester 8.0 25 A 9.4 60 both:0.4 acryl. acid ester 8.0 26 A 9.4 30 both: 0.05 acryl. acid ester 8.027 A 9.4 30 both: 2.3 acryl. acid ester 8.0 28 A 9.4 30 one: 2.3 acryl.acid ester 8.0 1) Si content was practically identical in bath and inaluminizing layer. 2) Resin/chromium ratio based on cured weight ratioof resin/chromium; chromium based on metallic chromium. 3) Underlinedvalues are outside of range of the invention.

TABLE 4 Press Weldability Corrosion Overall No. Sheet formability SpotSeam resistance evaluation Invention 1 A ◯ ◯ ◯ ◯ ⊚ Exs. 2 B ◯ ◯ ◯ ◯ ⊚ 3A ◯ ◯ ◯ ◯ ⊚ 4 A ◯ ◯ ◯ ◯ ⊚ 5 A ◯ Δ Δ ◯ ◯ 6 A ◯ ◯ ◯ ◯ ⊚ 7 A ◯ ◯ ◯ ◯ ⊚ 8 A◯ Δ Δ ◯ ⊚ 9 A ◯ ◯ ◯ ◯ ⊚ 10 A ◯ ◯ ◯ ◯ ⊚ 11 A ◯ ◯ ◯ ◯ ⊚ 12 A ◯ ◯ ◯ ◯ ⊚ 13A ◯ ◯ ◯ ◯ ⊚ 14 A ◯ ◯ ◯ ◯ ⊚ 15 A ◯ ◯ ◯ ◯ ⊚ 16 A ◯ ◯ ◯ ◯ ⊚ 17 A ◯ ◯ ◯ ◯ ⊚18 A Δ ◯ ◯ Δ ◯ 19 A ◯ Δ Δ ◯ ◯ 20 A ◯ ◯ ◯ ◯ ⊚ 21 A ◯ ◯ ◯ ◯ ⊚ 22 A ◯ Δ Δ ΔΔ Comp. 23 A x ◯ ◯ ◯ x Exs. 24 A Δ ◯ ◯ x x 25 A Δ x x ◯ x 26 A ◯ x x Δ x27 A ◯ x x ◯ x 28 A ◯ x x ◯ x * Overall evaluation ⊚: very excellent ◯:excellent Δ: somewhat inferior but usable x: unusable

Examples 29-50

Sheets comprising the components listed in Table 1 were used tofabricate cold-rolled sheets in the same manner as Example 1, and thesewere aluminized in the same manner as Example 1.

The aluminized steel sheets thus fabricated were coated with a chromatetreatment solution having one of the compositions listed in Table 5 to aprescribed the amount of coating using a roll coater or a linger rollafter immersion, and were then baked and dried with hot air at 150° C.

TABLE 5 Composition of chromate treatment bath by process of theinvention Chromic acid Organic polymer Phosphoric acid Conc. compoundcompound Silica Phosphonic acid Example Reduced chromic acid 20 g/lPolyacrylic acid Phosphoric acid Colloidal — Solution Cr³⁺/Cr⁶⁺ 5 g/l 60g/l silica C 5.5/4.5 10 g/l Example Reduced chromic acid 60 g/lPolyacrylic acid Phosphoric acid Colloidai 1-hydroxyethylidene- SolutionCr³⁺/Cr⁶⁺ 3 g/l 60 g/l silica 1,1-diphosphonic acid D 5.0/5.0 60 g/l 3.0g/l Exampie Reduced chromic acid 20 g/l Epoxy-acrylic acid Phosphoricacid Colloidal 1-hydroxyethylidene- Solution Cr³⁺/Cr⁶⁺ copolymer 60 g/lsilica 1,1-diphosphonic acid E 5.5/4.5 20 g/l 60 g/l 5.0 g/l ExampleReduced chromic acid 20 g/l Polyamine acrylic Phosphoric acid ColloidalSolution Cr³⁺/Cr⁶⁺ acid 20 g/l silica F 5.8/4.2 90 g/l 30 g/l ExampleChromic anhydride 20 g/l Polyacrylic acid Phosphoric acid Colloidal1-hydroxyethylidene- Solution 60 g/l 60 g/l silica 1,1-diphosphonic acidG 60 g/l 1.5 g/l Example Reduced chromic acid 20 g/l Vinyl acetate/Phosphoric acid Colloidal 1-hydroxyethylidene- Solution Cr³⁺/Cr⁶⁺ethylene copolymer 60 g/l silica 1,1-diphosphonic acid H 5.5/4.5 30 g/l60 g/l 3.0 g/l Example Chromic anhydride 20 g/l Polyacrylic acidPhosphoric acid Colloidal Solution 100 g/l 10 g/l silica I 20 g/lExample Chromic anhydride 20 g/l — — Colloidal Solution silica J 60 g/l

The suitability of steel sheets fabricated in the manner described aboveas fuel tanks was evaluated by the following method.

Weldability {circle around (1)}: Spot weldability evaluation

Weldability {circle around (2)}: Seam weldability evaluation

Press formability: Cylindrical drawing test A

Corrosion resistance: Plated steel sheet test

The results are shown in Tables 6 and 7. Tables 6 and 7 show thatsatisfactory performance was exhibited by all of the examples.

Examples 29-44 were materials with satisfactory resistance weldabilityrequired for automobile fuel tanks and also excellent press formabilityand corrosion resistance, and they are therefore very promising as newfuel tank materials and represent a major contribution to industry, as asolution to future difficulties involved with using Pb-based materialswhich have become an environmental problem.

TABLE 6 Performance evaluation results (for treatment on both sides)Amount Ratio of of Cr in Corro- resin to chrom- Press sion OverallTreatment Cr (*1) ate film Weldability forma- resist- evalua- No. Sheetsolution (wt %) (mg/m²) Spot Seam bility ance tion Present 29 A solutionC 50 10 ◯ ◯ ◯ ◯ ◯ inven- 30 A solution C 50 30 ◯ ◯ ◯ ⊚ ◯ tion 31 Asolution C 50 72 ⊚ ◯ ◯ ⊚ ⊚ 32 A solution C 50 120 ⊚ex ◯ ◯ ⊚ ⊚ 33 Asolution C 50 135 ⊚ex ◯ ◯ ⊚ ⊚ 34 A solution C 50 200 ⊚ ◯ ◯ ⊚ ⊚ 35 Bsolution C 50 75 ⊚ ◯ ◯ ⊚ ⊚ 36 A solution D 5 110 ⊚ex ◯ ◯ ⊚ ⊚ 37 Asolution E 200 80 ⊚ex ◯ ◯ ⊚ ⊚ 38 A solution F 450 25 ◯ ◯ ◯ ⊚ ◯ 39 Asolution G 300 75 ⊚ ◯ ◯ ⊚ ⊚ 40 A solution H 150 30 ⊚ ◯ ◯ ⊚ ⊚ Comp. 41 Asolution C 50 4 Δ x ◯ Δ Δ Exs. 42 A solution C 50 250 Δ x ◯ ⊚ Δ 43 Asolution I 500 20 Δ x ◯ Δ Δ 44 A solution J 0 70 Δ ◯ ◯ ⊚ Δ *Overallevaluation ⊚: Very excellent ◯: Excellent Δ: Somewhat inferior butusable x: Unusable *1: Weight ratio of added resin to chromic acid interms of Cr content.

TABLE 7 Amount Treat- Ratio of of Cr in Corro- ment resin to chrom-Press sion Overall solution Cr (*2) ate film Weldability forma- resist-evalu- No. Sheet (*1) (wt %) (mg/m²) Spot Seam bility ance tion Present45 A solution C/ 50/0 25/15 ◯ ◯ ⊚ ⊚ ◯ inven- solution J tion 46 Asolution C/ 50/0 75/20 ⊚ ◯ ⊚ ⊚ ⊚ solution J 47 A solution C/ 50/0135/100 ⊚ ◯ ⊚ ⊚ ⊚ solution J 48 A solution E/ 200/0 85/20 ⊚ ◯ ⊚ ⊚ ⊚solution J Comp. 49 A solution C/ 50/0 4/4 x x ⊚ Δ x Exs. solution J 50A solution C/ 50/0 250/160 Δ ◯ ⊚ ⊚ Δ solution J Overall evaluation ⊚:Very excellent ◯: Excellent Δ: Somewhat inferior but usable x: usable :Indicates combination of one side/one side : Weight ratio of added resinto chromic acid in terms of Cr content.

Examples 51-61

Sheets comprising the components listed in Table 8 were used tofabricate cold-rolled sheets in the same manner as Example 1, and thesewere subjected to hot dip aluminizing in the same manner as Example 1.

TABLE 8 Plated sheet components (wt %) Sample C Si Mn P S Ti Al B N C0.0011 0.03 0.31 0.007 0.009 0.056 0.04 0.0002 0.0033 D 0.00200 0.090.32 0.008 0.011 0.040 0.04 — 0.0032

The aluminized steel sheets thus fabricated were immersed in a chromatetreatment solution comprising 20 g/l CrO₃ and 60 g/l SiO₂, and theamount of coating was adjusted with a linger roll. They were then driedwith hot air at 80° C.

The suitability of steel sheets fabricated in the manner described aboveas fuel tanks was evaluated by the following method.

Press formability: Cylindrical drawing test A

Weldability {circle around (1)}: Spot weldability evaluation

Weldability {circle around (2)}: Brazing material spread

Corrosion resistance: Plated steel sheet test

The results are shown in Table 9. As seen in Table 9, when the amount ofchromate film is too low satisfactory corrosion resistance cannot beobtained and the weldability is inferior. Conversely, when the amount ofcoating is too high the brazing material wettability is reduced.

The present invention materials have satisfactory corrosion resistanceand press formability required for automobile fuel tanks and alsosuitability for a wide range of welding processes, and are thereforevery promising as new fuel tank materials and represent a majorcontribution to industry, as a solution to future difficulties involvedwith using Pb-based materials which have become an environmentalproblem.

TABLE 9 Amount of Cr in Weldability Corro- chrom- Press Brazing sionOverall ate film form- material resist- eval- Ex. Sheet (mg/m²) abilitySpot spread ance uation Examples 51 C 10 ◯ ◯ ⊚ ◯ ◯ 52 C 18 ◯ ◯ ⊚ ◯ ◯ 53C 20 ◯ ◯ ⊚ ⊚ ⊚ 54 D 20 ◯ ◯ ⊚ ⊚ ⊚ 55 C 30 ◯ ◯ ⊚ ⊚ ⊚ 56 C 31 ◯ ◯ ◯ ⊚ ◯ 57C 34 ◯ ◯ ◯ ⊚ ◯ Comp. 58 C 35 ◯ ◯ Δ ⊚ Δ Exs. 59 C 40 ◯ ◯ Δ ⊚ Δ 60 C 5 ◯ Δ⊚ Δ Δ 61 C 70 ◯ ◯ x ⊚ Δ *Overall evaluation ⊚: Very excellent ◯:Excellent Δ: Somewhat inferior but usable x: Unusable

Examples 62-90

Plating sheets comprising the components listed in Table 8 were used tofabricate cold-rolled sheets in the same manner as Example 1, and thesewere subjected to hot dip aluminizing in the same manner as Example 1.

Aluminized steel sheets produced in this manner were subjected tocomposite chromate treatment and inorganic chromate treatment with thebaths of Tables 10 and 11 as the standard compositions. The filmthicknesses (Cr amount of coatings) of both chromate films were adjustedby linger roll, and hot air at 80° C. was used for drying to completethe film.

The organic film treatment was a baking type commonly employed for epoxyresins, acrylic resins and polyethylene resins.

TABLE 10 Lubricant-containing composite chromate treatment solutioncomposition Composite chromate treatment solution concentration Resin60-180 g/l Chromic acid 5-60 g/l Phosphoric acid 10-60 g/l Colloidalsilica 5-20 g/l Lubricant 0.1-50 g/l

TABLE 11 Inorganic chromate treatment solution composition Inorganicchromate treatment solution concentration Chromic acid 10-100 g/lPhosphoric acid 0-60 g/l (containing organic phosphoric acid) Colloidalsilica 15-250 g/l

The performance of steel sheets produced in this manner as fuel tankswas evaluated. The evaluation method used here was the following. Theplating conditions and performance evaluation results are shown in Table12.

Press formability {circle around (1)}: Cylindrical drawing test B

Press formability {circle around (2)}: Bowden friction coefficientmeasurement

Corrosion resistance: Plated steel sheet test

Examples 62-90 provided hot dip aluminized steel sheets with the pressformability and corrosion resistance required for automobile fuel tanksand also excellent welding properties, and they are therefore verypromising as new fuel tank materials and represent a major contributionto industry, as a solution to future difficulties involved with usingPb-based materials which have become an environmental problem.

TABLE 12 Amount Composite chromate film Opposite side Si content of Filmtreatment for Press Corro- of plating plating thickness Lubricantone-sided formability sion Overall layer¹⁾ per side per side Main AmountSides composite Cylin. Bowden resis- evalua- No. Sheet (wt %) (g/m²)(μm) resin Type (wt %) treated chromate films drawing friction tancetion²⁾ Exs. 62 C 9.4 30 0.4 acrylic stearic 0.5 both ◯ ◯ ⊚ ◯ acid acidester 63 D 9.4 30 0.4 acrylic stearic 1 ⊚ ⊚ ⊚ ⊚ acid acid ester 64 C 5.230 0.4 acrylic stearic 5 ⊚ ⊚ ⊚ ⊚ acid acid ester 65 C 11.4 30 0.4acrylic stearic 10 ⊚ ⊚ ⊚ ⊚ acid acid ester 66 C 0.4 30 0.4 acrylicstearic 20 ⊚ ⊚ ⊚ ⊚ acid acid ester 67 C 9.4 30 0.2 acrylic ester 10 ⊚ ⊚⊚ ⊚ acid ester 68 C 9.4 30 0.8 acrylic ester 5 one CrO₃—SiO₂- ◯ ⊚ ⊚ ◯acid based inorganic ester chromate 69 C 9.4 30 1.2 acrylic silicon 1both ⊚ ⊚ ⊚ ⊚ acid ester 70 C 9.4 30 0.2 acrylic silicon 3 ⊚ ⊚ ⊚ ⊚ acidester 71 C 9.4 30 0.4 acrylic silicon 8 ⊚ ⊚ ⊚ ⊚ acid ester 72 C 9.4 300.8 acrylic silicon 15 ⊚ ⊚ ⊚ ⊚ acid ester 73 C 9.4 30 1.2 acrylicparaffin 5 one Epoxy film, ⊚ ⊚ ⊚ ⊚ acid brazing 1μ application esterfiller 74 C 9.4 30 1.8 acrylic special both ⊚ ⊚ ⊚ ⊚ acid olefin ester 75C 9.4 30 0.4 vinyl special 3 ⊚ ⊚ ⊚ ⊚ carb- olefin oxylate ester 76 C 9.430 0.4 vinyl special 8 ⊚ ⊚ ⊚ ⊚ ether olefin 77 C 9.4 30 0.4 styrenespecial 12 ⊚ ⊚ ⊚ ⊚ olefin 78 C 9.4 30 0.4 acryla- stearic 0.5 ◯ ◯ ⊚ ◯mide acid 79 C 14.5 30 0.4 epoxy stearic 1 ⊚ ⊚ ⊚ ⊚ acid 80 C 9.4 65 0.4acrylic stearic 5 ⊚ ⊚ ⊚ ◯³⁾ acid acid ester 81 C 9.4 60 0.4 acrylicstearic 10 ⊚ ⊚ ⊚ ⊚ acid acid ester 82 C 9.4 20 0.4 acrylic stearic 18 ⊚⊚ ⊚ ⊚ acid acid ester Comp. 83 C 1.5 30 0.4 acrylic not 0 x Δ x x Exs.acid added ester 84 C 16.0 30 0.4 acrylic stearic 0.3 Δ ◯ x x acid acidester 85 C 9.4 70 0.4 acrylic not 0 x Δ Δ x acid added ester 86 C 9.4 300.05 acrylic not 0 x x x x acid added ester 87 C 9.4 30 2.3 acrylicstearic 0.3 Δ Δ ⊚ x(poor acid acid weld- ester ing) 88 C 9.4 65 0.08acrylic not 0 x x Δ x acid added ester 89 C 9.4 30 0.05 epoxy not 0 x xΔ x added 90 C 9.4 30 2.3 acrylic not 0 Δ Δ ⊚ x(poor acid added weld-ester ing) ^(*1))Plating layer Si content = Si/Al + Si (wt %),determined by chemical analysis. ^(*2))Weldability also considered inoverall evaluation, press property, corrosion resistance; ⊚: veryexcellent, ◯: excellent, Δ: somewhat poor but usable, x: unusable.^(*3))Weldability somewhat poor, but usable.

Examples 91-119

Sheets comprising the components listed in Table 8 were used tofabricate cold-rolled sheets in the same manner as Example 1, and thesewere subjected to hot dip aluminizing in the same manner as Example 1.

Aluminized steel sheets produced in this manner were subjected toinorganic-based chromate treatment and composite chromate treatment withthe bath of Table 13 as the standard composition. The amount of chromatefilm and composite chromate film thicknesses were adjusted by lingerroll, and hot air at 80° C. was used for drying to complete each film.

TABLE 13 Compositions of inorganic-based chromate films and resinchromate treatment solutions Inorganic-based chromate treatmentComposite chromate solution treatment solution concentrationconcentration Resin — 60-180 g/l Chromic acid 15-50 g/l 5-60 g/lPhosphoric acid 10-30 g/l 10-60 g/l Colloidal silica 10-200 g/l 5-20 g/l

The performance of steel sheets produced in this manner as fuel tankswas evaluated by the following method. The treatment conditions andperformance evaluation results are shown in Table 17.

Press formability: Cylindrical drawing test A

Weldability: Spot weldability evaluation

Corrosion resistance: Plated steel sheet test

Corrosion resistance: Flaw corrosion resistance test

As shown in Table 14, when the Si content of the plating is too low(Comparative Example 113) the alloy layer grows too much, resulting inpeeling of the plating during working. Conversely, when the Si contentis too high (Comparative Example 114), the corrosion resistance isimpaired. When the amount of aluminum plating is too great (ComparativeExample 117) the welding section is inferior. When the compositechromate film thickness is too small (Comparative Examples 115, 177) ortoo large (Comparative Examples 116, 119), satisfactory weldabilitycannot be obtained. Satisfactory weldability also cannot be obtainedwhen the inorganic-based chromate film thickness is too large(Comparative Example 118).

Examples 91-119 provide hot dip aluminized steel sheets which have boththe corrosion resistance and press formability required for automobilefuel tank materials, as well as achieving improved weldability which hasbeen a problem in the past, and they are therefore very promising as newfuel tank materials and represent a major contribution to industry, as asolution to future difficulties involved with using Pb-based materialswhich have become an environmental problem.

Regarding the amount of the composite chromate films in Examples 91-119,with a Cr content of less than 10 mg/m² the effect of corrosionresistance is insufficient, raising concerns of corrosion in platinglayer cracks during working. Also, the plating metal tends to adhere tothe electrode during spot welding, thus impairing continuous operation.An amount of 10 mg/m² or greater gives good corrosion resistance andresistance weldability as a fuel tank, but at 80 mg/m² or greater theresistance weldability is even better. On the other hand, if the amountof coating exceeds 200 mg/m² the corrosion resistance is satisfactorybut the increased resistance value between the steel sheets due to thelarge film thickness results in poor electrization (electric currentpassing) and local overelectrization, creating problems such as poorercontinuous operation. The amount of coating is preferably no greaterthan 140 mg/m². From this viewpoint, therefore, the present inventorsdetermined the range to be from 10 mg/m² to 200 mg/m², and morepreferably from 80 mg/m² to 140 mg/m².

TABLE 14 Inorganic-based chromate film Amount of Inorganic-based betweencomposite chromate and Si content Al-based chromate film side platinglayer of plating plating Amount of Amount of Ex. layer¹⁾ per sidechromate film Composite chromate film side Chromate No. Sheet (wt %)(g/m²) (mg/m²) Type Main resin film (mg/m²) Type Inven-  91 C 9.4 30 15CrO₃—SiO₂- 0.4 acrylic acid ester — — tion  92 D 9.4 30 15 based 0.4acrylic acid ester — — Exs.  93 C 5.2 30 20 0.4 acrylic acid ester — — 94 C 11.4 30 20 0.4 acrylic acid ester — —  95 C 9.4 45 50 0.4 acrylicacid ester — —  96 C 9.4 30 50 CrO₃—SiO₂- 0.4 acrylic acid ester  15CrO₃—SiO₂-based  97 C 9.4 30 50 phosphoric 0.4 acrylic acid ester  15CrO₃—SiO₂-based  98 C 9.4 30 75 acid-based 1.2 acrylic acid ester  20CrO₃—SiO₂-based  99 C 9.4 30 120 0.2 acrylic acid ester  60 CrO₃—SiO₂-phosphoric acid-based 100 C 9.4 30 200 0.4 acrylic acid ester 100CrO₃—SiO₂-organic phosphoric acid-based 101 C 9.4 30 50 CrO₃—SiO₂- 0.8acrylic acid ester — — 102 C 9.4 30 50 organic 1.2 acrylic acid ester —— 103 C 9.4 30 75 phosphoric 1.8 acrylic acid ester — — 104 C 9.4 30 75acid-based 0.4 vinyl carboxylate ester — — 105 C 9.4 30 75 0.4 vinylether — — 106 C 9.4 30 100 CrO₃—SiO₂- 0.4 styrene — — 107 C 9.4 30 100based 0.4 acrylamide — — 108 C 14.5 30 150 0.4 epoxy — — 109 C 9.4 65 15CrO₃—SiO₂- 0.4 acrylic acid ester — — 110 C 9.4 60 15 acrylic 0.4acrylic acid ester — — 111 C 9.4 70 5 resin-based 0.4 acrylic acid ester— — 112 C 9.4 20 10 0.4 acrylic acid ester — — Comp. 113 C 1.5 30 — —0.4 acrylic acid ester — — Exs. 114 C 16.0 30 15 CrO₃—SiO₂- 0.4 acrylicacid ester — — 115 C 9.4 30 20 based 0.05 acrylic acid ester — — 116 C9.4 30 — — 2.3 acrylic acid ester — — 117 C 9.4 65 15 CrO₃—SiO₂- 0.08acrylic acid ester — — 118 C 9.4 30 250 based 0.4 epoxy — — 119 C 9.4 3015 2.3 acrylic acid ester 140 CrO₃—SiO₂-based Corrosion resistance Flawcorrosion resisitance Combination Evaluated side: Evaluated side:Evaluated side: Evaluated side: Ex of steel sheets press Spot compositeinorganic-based composite inorganic-based Overall No. for welding²⁾formability weldability chromate side chromate side chromate sidechromate side evaluation Invention  91 C ⊚ ⊚ ⊚ ⊚ ◯ ◯ex ⊚ Exs.  92 C ⊚ ⊚⊚ ⊚ ◯ ◯ex ⊚  93 C ⊚ ⊚ ⊚ ⊚ ◯ ◯ex ⊚  94 C ⊚ ⊚ ⊚ ⊚ ◯ ◯ex ⊚  95 C ⊚ ⊚ ⊚ ⊚ ◯◯ex ⊚  96 D ⊚ ⊚ ⊚ ⊚ ◯ex ◯ex ⊚  97 E ⊚ ⊚ ⊚ ⊚ ◯ex ◯ex ⊚  98 C ⊚ ⊚ex ⊚ ⊚◯ex ◯ex ⊚  99 C ⊚ ⊚ex ⊚ ⊚ ◯ex ◯ex ⊚ 100 C ⊚ ⊚ ⊚ ⊚ ◯ex ◯ex ⊚ 101 C ⊚ ⊚ ⊚⊚ ◯ ◯ex ⊚ 102 C ⊚ ⊚ ⊚ ⊚ ◯ ◯ex ⊚ 103 C ⊚ ⊚ex ⊚ ⊚ ◯ ◯ex ⊚ 104 C ⊚ ⊚ex ⊚ ⊚◯ ◯ex ⊚ 105 C ⊚ ⊚ex ⊚ ⊚ ◯ ◯ex ⊚ 106 C ⊚ ⊚ex ⊚ ⊚ ◯ ◯ex ⊚ 107 C ⊚ ⊚ex ⊚ ⊚◯ ◯ex ⊚ 108 C Δ ⊚ Δ Δ ◯ ◯ex ◯ 109 C ⊚ ◯ ⊚ ⊚ ◯ ◯ex Δ 110 C ⊚ ⊚ ⊚ ⊚ ◯ ◯ex⊚ 111 C ⊚ ◯ ⊚ Δ ◯ ◯ Δ 112 C ⊚ ⊚ ⊚ ⊚ ◯ ◯ ◯ 113 C X ⊚ ⊚ Δ ◯ Δ X Comp. Exs.114 C ◯ ⊚ X X ◯ Δ X 115 C ⊚ X X ⊚ X ◯ex X 116 C ⊚ X ⊚ Δ ◯ Δ X 117 C ◯ X⊚ ⊚ Δ ◯ X 118 C ⊚ X ⊚ ⊚ ◯ ◯ex X 119 C ⊚ X ⊚ ⊚ ◯ex ◯ex X ¹⁾Plating layerSi content = Si/Al + Si (wt %), determined by chemical anaylsis. ²⁾Steelsheet combination: Composite chromate treated side between steel sheets:C, Inorganic-based chromate treated side: D, One side composite chromatetreated side, other side inorganic chromate treated side: E *Overallevaluation ⊚: very excellent, ◯: excellent, Δ: somewhat poor but usable,x: unusable

Examples 120-141

Actual fuel tanks come in a variety of different shapes, and are notstandardized. Thus, several types of differently worked fuel tanks wereproduced using as materials hot dip aluminized steel sheets (thickness:0.8 mm) with different steel components, plating compositions and resinfilms, and Pb—Sn alloy-plated steel sheets. The Pb content was 0.001% asthe impurity in the aluminizing layers of the hot dip aluminized steelsheets. The sheet thickness reduction was evaluated for quantificationof the extent of working. The extent of working was evaluated as themaximum value of the sheet thickness reduction calculated upon measuringthe sheet thickness at each site before and after forming. Spot weldingand brazing were used for joining of the details after forming. Thesteel components of the materials used are listed in Table 15, the resinfilm descriptions in Table 16 and the fuel tank production conditions inTable 17.

TABLE 15 Components of plated sheets (wt %) Sample C Si Mn P S Ti Al B NE 0.0042 0.09 0.30 0.008 0.012 0.03 0.05 0.0002 0.0033 F 0.0009 0.030.32 0.007 0.011 0.03 0.04 0.0002 0.0032

TABLE 16 Material description Amount of Resin plating on Amount ofthick- Tank Plating one side After- Treated Resin Cr ness Added materialSheet composition (g/m²) treatment sides system (mg/m²) (μm) componentsA E Al-10% Si 30 composite both acrylic 30 0.3 phosphoric B F Al-10% Si30 chromate 30 0.3 acid, C E Al-6% Si  30 30 0.3 silica D E Al-12% Si 3030 0.3 E E Al-10% Si 20 30 0.3 F E Al-10% Si 50 30 0.3 G E Al-10% Si 2040 0.4 H E Al-10% Si 20 60 0.6 I E Al-10% Si 20 20 0.4 J E Al-10% Si 30resin 20 1.0 silica K E Al-10% Si 30 coating polyether 20 1.0 L E Al-10%Si 30 epoxy 20 1.0 M E Al-10% Si 30 acrylic 10 1.0 N E Al-10% Si 30 200.6 O E Al-10% Si 30 20 1.8 P E Al-10% Si 30 20 1.0 none Q E Al-10% Si30 composite one 30 0.3 phosphoric chromate acid, silica R E Al-10% Si30 chromate both —  5 — — S E Pb-8% Sn 40 phosphate — — — — T E Zn-10%Ni 20 chromate — 50 — — 1) Amount of coating, Cr amount of coatingindicated per side. 2) Cr amount of coating: value in terms of metallicCr. 3) Resin coating: resin coating after chromate.

TABLE 17 Fuel tank production conditions and performance Sheet thicknessPb Overall Mate- Resin reduction Corrosion elu- evalu- Ex. No. rial side% resistance tion ation Invention 120 A — 20 ⊚ ◯ ◯ examples 15 ⊚ ◯ 8 ⊚ ◯121 A — 20 ⊚ ◯ ◯ 122 B — 20 ⊚ ◯ ◯ 123 C — 20 ⊚ ◯ ◯ 124 D — 20 ⊚ ◯ ◯ 125E — 20 ⊚ ◯ ◯ 126 F — 20 ⊚ ◯ ◯ 127 G — 20 ⊚ ◯ ◯ 128 H — 20 ⊚ ◯ ◯ 129 I —20 ⊚ ◯ ◯ 130 J — 20 ⊚ ◯ ◯ 131 K — 20 ⊚ ◯ ◯ 132 L — 20 ⊚ ◯ ◯ 133 M — 20 ⊚◯ ◯ 134 N — 20 ⊚ ◯ ◯ 135 0 — 20 ⊚ ◯ ◯ 136 P — 20 ⊚ ◯ ◯ 137 Q inner 20 ⊚◯ ◯ 138 Q outer 20 ⊚-Δ ◯ ◯ 20 X ◯ X Comp. 139 R — 15 Δ ◯ examples 8 ⊚ ◯140 S — 20 ⊚ X X 141 T — 20 X ◯ X Overall evaluation: ◯: excellent, X:unsuitable

The corrosion resistance and Pb elution were evaluated under thefollowing conditions.

Weldability {circle around (1)}—Fuel tank test

Weldability {circle around (2)}—Pb elution

As shown in Table 17, the fuel tanks fabricated with aluminizing layersincluding no resin film had thick chromate coatings and thereforeexhibited some degree of corrosion resistance with minimally workedshapes, but the corrosion resistance was worse with shapes of higherworking to a sheet thickness reduction of 15% or greater, such as iscommon with actual fuel tanks (Comparative Example 139). While thecorrosion resistance was satisfactory with the fuel tank employing aconventionally used Pb—Sn plated steel sheet (Comparative Example 140)and the one employing Pb—Sn based solder on an aluminized steel sheet(Comparative Example 141), Pb elution was a concern. The corrosionresistance was notably poor with the fuel tank made of a material coatedwith Zn—Ni chromate. When a material having a resin film on analuminizing layer was formed and an Al-based brazing material materialwas used, there was no concern of Pb elution and a fuel tank withexcellent corrosion resistance after working was obtained. However,Example 135 required some change in pressure force and current value forwelding, which impeded productivity during welding.

Examples 120-141 eliminated concerns of Pb contamination of theenvironment which has become a problem recently, and provided fuel tankswith excellent corrosion resistance even under forming into complexshapes. They also represent a major contribution to industry as aresponse to increasing calls for environmental conservation.

Examples 142-155

Cold-rolled steel sheets fabricated according to Example 1 using sheetswith the composition listed in Table 18 were coated with hot dipaluminizing on both sides in the manner of Example 1. One of the sidesof each aluminized material was also subjected to Belder grinding toprepare a one side-coated material.

TABLE 18 Plated sheet composition (wt %) Sample C Si Mn P S Ti Al B N G0.0011 0.03 0.31 0.007 0.009 0.054 0.04 0.0002 0.0033

Each aluminized steel sheet thus fabricated was coated with one ofdifferent treatment solutions to a prescribed amount of coating using aroll coater or a linger roll after immersion, and was then baked anddried with hot air at 200° C. The seam weldability of these resin-coatedaluminized steel sheets was evaluated by the following method.

Weldability—seam weldability evaluation

The results are listed in Table 19. As seen in Table 19, all of theexamples exhibited satisfactory seam weldability.

Examples 142-155 provided seam welding methods required for automobilefuel tank materials, and they are therefore very promising as new fueltank materials and represent a major contribution to industry, as asolution to future difficulties involved with using Pb-based materialswhich have become an environmental problem.

In examples 142-155, the chromic acid addition amounts are notparticularly restricted but are best at from 10 mg/m² to 200 mg/m² interms of Cr. At less than 10 mg/m² the effect of addition isinsufficient, and with an amount of 10 mg/m² or greater the fuel tankhas good corrosion resistance and resistance weldability, but theresistance weldability is even better at greater than 70 mg/m². On theother hand, if the amount of coating is greater than 200 mg/m² theproportion of inorganic matter in the film increases, and thereforedespite satisfactory corrosion resistance there will be problems oflocal overelectrization and reduced continuous operation. The amount ofcoating is preferred to be no greater than 140 mg/m². From thisviewpoint, therefore, the range was determined to be from 10 mg/m² to200 mg/m², and more preferably from 80 mg/m² to 140 mg/m².

TABLE 19 Coating conditions and evaluation results Upper sheet Lowersheet Amount of Amount of plating Resin plating Resin (electrode film(electrode film side/steel Resin coating conditions thick- side/steelResin coating conditions thick- Seam Ex. sheet side (electrodeside/steel sheet side) ness sheet side (electrode side/steel sheet side)ness weld- No. (g/m²) (g/m²) (μm) (g/m²) (g/m²) (μm) ability Present 14240/40 epoxy resin/epoxy resin 0.5 40/40 epoxy resin/epoxy resin 0.5 ⊚inven- 143 30/30 none/epoxy resin 0.5 30/30 none/none — Δ tion 144 30/30epoxy resin/none 0.5 none/30  none/none — ⊚ 145 30/30 acrylicresin/acrylic resin 0.5 none/30  acrylic resin/acrylic resin 0.5 ⊚ 146none/10  epoxy resin/epoxy resin 0.1 none/10  epoxy resin/epoxy resin0.1 ⊚ 147 30/30 chromate composite acrylic 0.5 30/30 chromate compositeacrylic 0.5 ◯ resin/chromate composite acrylic resin/chromate compositeacrylic resin chromate content (Cr resin chromate content (Cr content):20 mg/m² content): 20 mg/m² 148 30/30 chromate composite acrylic 0.530/30 chromate composite acrylic 0.5 ⊚ resin/chromate composite acrylicresin/chromate composite acrylic resin chromate content (Cr resinchromate content (Cr content): 80 mg/m² content): 80 mg/m² 149 30/30chromate composite acrylic 0.5 30/30 chromate composite acrylic 0.5 ⊚resin/chromate composite acrylic resin/chromate composite acrylic resinchromate content (Cr resin chromate content (Cr content): 140 mg/m²content): 140 mg/m² 150 30/30 chromate composite acrylic 0.5 30/30chromate composite acrylic 0.5 ◯ resin/chromate composite acrylicresin/chromate composite acrylic resin chromate content (Cr resinchromate content (Cr content): 200 mg/m² content): 200 mg/m² 151 30/30chromate composite acrylic 0.5 30/30 chromate composite acrylic 0.5 Δresin/chromate composite acrylic resin/chromate composite acrylic resinchromate content (Cr resin chromate content (Cr content): 250 mg/m²content): 250 mg/m² 152 60/60 epoxy resin/epoxy resin 0.5 60/60 epoxyresin/epoxy resin 0.5 Δ 153 50/50 polyethylene resin/polyethylene 2.050/50 polyethylene resin/polyethylene 2.0 ◯ resin resin Comp. 154 50/50chromate film/chromate film — 50/50 chromate film/chromate film — X Exs.(Cr content: 20 mg/m²) (Cr content: 20 mg/m²) 155 30/30 — 0.5 30/30 —0.5 X

What is claimed is:
 1. A coated aluminized steel sheet suitable for fueltanks, which consists of, in combination: (a) a steel sheet, (b) analuminizing layer formed on one or both sides of said steel sheet andbased on aluminum or an aluminum alloy containing 2-15 wt % silicon, and(c) a coating layer formed on at least one of said aluminizing layers,said coating layer being one or more films selected from the groupconsisting of: i) an organic and inorganic composite chromate filmhaving a film thickness of 0.1-2 μm and containing a resin and a chromicacid compound, with the re sin/metal chromium weight ratio in the rangeof 0.5-18, ii) an inorganic-based chromate film A with the coating layerformed to 10-200 mg/m² in terms of metallic chromium, which comprises100 parts by weight of chromic acid in terms of metallic chromium and100-1000 parts by weight of colloidal silica, and further comprises atleast one selected from the group consisting of 100-600 parts by weightof a phosphoric acid compound, 10-200 parts by weight of a phosphonicacid or phosphonic acid salt compound and less than 50 parts by weightof an organic resin, and iii) an inorganic-based chromate film B with acoating amount of at least 10 mg/m² and less than 35 mg/m² in terms ofmetallic chromium.
 2. A coated aluminized steel sheet according to claim1, wherein said aluminizing layer is formed to 60 g/m² or less.
 3. Acoated aluminized steel sheet according to claim 1, wherein saidcomposite chromate film further contains 0.5-20 wt % of a lubricant. 4.A coated aluminized steel sheet according to claim 1, wherein saidcomposite chromate film further contains 100-600 parts by weight of aphosphoric acid compound and 100-1000 parts by weight of colloidalsilica with respect to 100 parts by weight of metallic chromium.
 5. Acoated aluminized steel sheet according to claim 4, wherein saidcomposite chromate film further contains 10-200 parts by weight of aphosphonic acid or phosphonic acid salt compound with respect to 100parts by weight of metallic chromium.
 6. A coated aluminized steel sheetaccording to claim 1, which has said aluminizing layer on both sides ofsaid steel sheet and which has said composite chromate film on thealuminizing layers on both sides.
 7. A coated aluminized steel sheetaccording to claim 1 which has said aluminizing layer on both sides ofsaid steel sheet, and which has said inorganic-based chromate film A onsaid aluminizing layers on both sides.
 8. A coated aluminized steelsheet according to claim 1, which has said aluminizing layer on bothsides of said steel sheet and which has said composite chromate film onsaid aluminizing layer on one side and an inorganic-based chromate filmC with a coating amount of 200 mg/m² or less in terms of metallicchromium on said aluminizing layer on the other side.
 9. A coatedaluminized steel sheet according to claim 8, wherein saidinorganic-based chromate film C formed on said aluminizing layer furthercontains at least one selected from the group consisting of phosphoricacid compounds, phosphonic acid and phosphonic acid salt compounds, andless than 50 parts by weight of a resin with respect to 100 parts byweight of metallic chromium.
 10. A coated aluminized steel sheetaccording to claim 1, which has an inorganic-based chromate film C witha coating amount of 100 mg/m² or less in terms of metallic chromiumbetween said aluminizing layer and said composite chromate film.
 11. Acoated aluminized steel sheet according to claim 10, wherein saidinorganic-based chromate film C formed between said aluminizing layerand said composite chromate film further contains at least one selectedfrom the group consisting of phosphoric acid compounds, phosphonic acidand phosphonic acid salt compounds, and less than 10 parts by weight ofa resin with respect to 100 parts by weight of metallic chromium.
 12. Acoated aluminized steel sheet according to claim 1, which has saidaluminizing layer on both sides of said steel sheet, and which has saidinorganic-based chromate film B formed to 10-35 mg/M² in terms ofmetallic chromium on said aluminizing layers on both sides.
 13. A coatedaluminized steel sheet according to claim 1, which has said aluminizinglayer on both sides of said steel sheet, and which has said compositechromate film on said aluminizing layer on one side and an inorganicresin film with a thickness of 0.1-2.0 μm on said aluminizing layer onthe other side.
 14. A coated aluminized steel sheet according to claim13, which has an inorganic-based chromate film C with a coating amountof 100 mg/m² or less in terms of metallic chromium between saidaluminizing layer and at least one of said composite chromate film andsaid organic resin film.
 15. A coated aluminized steel sheet accordingto claim 14, wherein said inorganic-based chromate film C formed on saidaluminizing layer further contains at least one selected from the groupconsisting of phosphoric acid compounds, phosphonic acid and phosphonicacid salt compounds, and less than 50 parts by weight of a resin withrespect to 100 parts by weight of metallic chromium.
 16. A coatedaluminized steel sheet according to claim 1, which has said aluminizinglayer on both sides of said steel sheet and which has saidinorganic-based chromate film B on said aluminizing layer on one sideand an organic-based resin film on said aluminizing layer on t he otherside.
 17. A coated aluminized steel sheet according to claim 16, whereinsaid inorganic-based chromate film B is formed to 200 mg/m² in terms ofmetallic chromium.
 18. A coated aluminized steel sheet according toclaim 17, wherein said inorganic-based chromate film formed on saidaluminizing layer further contains at least one selected from the groupconsisting of phosphoric acid compounds, phosphonic acid and phosphonicacid salt compounds, and less than 50 parts by weight of a resin withrespect to 100 parts by weight of metallic chromium.
 19. A coatedaluminized steel sheet according to claim 17, which has aninorganic-based chromate film C with a coating amount of 100 mg/m² orless in terms of metallic chromium between said aluminizing layer andsaid organic resin film.
 20. A coated aluminized steel sheet accordingto claim 19, wherein said inorganic-based chromate film C formed betweensaid aluminizing layer and said organic resin film further contains atleast one selected from the group consisting of phosphoric acidcompounds, phosphonic acid and phosphonic acid salt compounds, and lessthan 5 parts by weight of a resin with respect to 100 parts by weight ofmetallic chromium.
 21. A fuel tank produced with a coating aluminizedsteel sheet according to claim 1 above.
 22. An automobile fuel tankwherein a pair of bowl-shaped bodies with flanges are integrated bycontinuous seam-welding of the flange substances, the automobile fueltank being characterized in that the materials of which said bowl-shapedbodies are made are coating aluminized steel sheets which consist ofaluminized steel sheets each having on one or both sides an aluminizinglayer based on aluminum or an aluminum alloy containing 2-13 wt %silicon, and having a resin coating on the uppermost surface of theinner and/or outer side.
 23. An automobile fuel tank according to claim22 above, wherein said resin coating is an organic and inorganiccomposite chromate film consisting of a mixture of a resin and a chromicacid compound.
 24. An automobile fuel tank according to claim 22 above,wherein said resin coating has a thickness of 0.1-2 μm.
 25. Anautomobile fuel tank according to claim 22, wherein said coatedaluminized steel sheets are coated aluminized steel sheets according toclaim
 1. 26. A seam welding process for fuel tanks, in which two coatedaluminized steel sheets are combined which are aluminized steel sheetseach having formed on one or both sides an aluminizing layer based onaluminum or an aluminum alloy containing 2-13 wt % silicon and having aresin coating formed on the one or both sides thereof, wherein saidcoated aluminized steel sheets have said aluminizing layer at least onthe side corresponding to the inner side of the fuel tank, a resin filmis provided on at least one of the steel sheet surfaces at the sidewhere the steel sheets meet and/or on at least one of the steel sheetsurfaces at the side where it contacts with an electrode wheel, and saidtwo combined steel sheets are then seam welded between a pair ofelectrode wheels.
 27. The process according to claim 26 above, whereinsaid resin film contains chromic acid at 10-200 mg/m² in terms of Cr.28. The process according to claim 27 above, wherein said resin film hasa thickness of 0.1-2 μm.
 29. The process according to claim 26 above,wherein said resin film formed on the surface of said aluminized steelsheet is an organic/inorganic composite chromate film according to claim1 above.
 30. The process according to claim 26, wherein said coatedaluminized steel sheet is a coated aluminized steel sheet according toclaim 1.